In no particular order:
1) Kevin Drum is right about many things, but most notably about this one:

[E]ven if the specific evidence in the State of the Union speech was dubious, what was the general prewar assessment of Saddam's nuclear bomb program? Should George Bush have been talking about it at all?
So I pulled my copy of The Threatening Storm off the shelf and reread the section on nuclear weapons (pp. 173-175). It's unequivocal: writing in late 2002, Kenneth Pollack says there is a "consensus" that Iraq has an active nuclear program; it employs as many as 14,000 workers; experts "unanimously" agree that Iraq is working to enrich uranium; and Iraq might be able to build a bomb as early as 2004.
But unlike chemical and biological weapons, which might yet be found, a nuclear program is too big to hide. If we haven't found it by now, it just doesn't exist, and that means that something that was "unanimously" agreed upon in late 2002 has turned out to be flatly wrong.
By the end of January, with UN inspectors roaming freely around Iraq, the evidence for a nuclear program was dwindling fast. For some reason, though, Bush's advisors felt that chemical and biological weapons weren't enough for his State of the Union speech, so they seized on what little was left in order to keep the threat of nuclear bombs alive. That's bad enough, but even worse is how the collective intelligence agencies of the world misjudged what was happening in Iraq so badly. This isn't a small point of interpretation, it's a case of absolute certainty about a massive technical and industrial program that turned out to be complete fiction.
How did that happen?
(Emphasis added.)

Leaving out the obvious caveats about how it is still far too early to say that Iraq's nuclear program is a "complete fiction" and "if we haven't found it by now, it doesn't exist" (after all, the first round of inspectors didn't find Iraq's nuclear program until directed to it by defector Hussein Kamel, four years after inspections began), Kevin is absolutely right that the Niger "gotcha" game is a stand-in for the real issue of whether the world's intelligence services completely misread the situation. And if so, that scandal far outstrips any question of whether a particular claim should or should not have been in the State of the Union address. (For one, it clearly pre-dates the Bush administration, so the question of whether they improperly bullied the CIA is irrelevant.) And on the tactical level, this conflation - clearly being encouraged by some administration critics - will likely backfire, as the storm over the second question will likely dissipate once the US finds some store of chemical weapons - which (I think) is still likely - and also defusing the "Bush lied" storyline.
2) I still think that administration critics like Josh Marshall are semi-willfully blinding themselves to the main message of the Administration's arguments for the war. And because of that, I think they are overstating the importance of the Niger/uranium claim.
3) The "Bush lied" string is clearly based, in large part, on resentment over the way Republicans treated President Clinton and the 2000 election, and the according desire for revenge. Don't believe me? Ask Michael Tomasky, showing signs of Kool-Aid overdose. I found this piece strangely gripping:

Here, distilled into four paragraphs, is the liberal interpretation of the last 10 years.
After a long and in some ways well-earned stroll in the wilderness, Democrats finally elect one of their own to the presidency. He is a prodigiously talented man. He has flaws, to be sure, and some of them are important. But far more important is the way the rules of the game change upon his ascension. On election night, the nation's leading Republican goes on television and snorts that the victory is illegitimate; from that point on, a campaign is waged to destroy -- not tarnish or discredit or soften up, but destroy -- the new president and his wife. This campaign has no precedent in American political history. (Please spare us the Alexander-Hamilton-and-his-mistress parallel; the 1790s are not parallel to today's world, and Hamilton was attacked by one yellow journalist, not a network of operatives with tens of millions of dollars to spend.) Finally, he is caught in flagrante. Even then, the public asserts directly and repeatedly that it does not consider the offense a high crime or misdemeanor.
But no matter. Against the clear will of the people, impeachment proceeds. It fails, but the hounding, again mostly over pseudo-scandals (like a West Wing ransacking) that never happened but are endlessly hyped by a frivolous media, continues. And in its way, this technique succeeds: What was objectively a bountiful and comparatively humane period in American history -- prosperity, peace, low crime, reduced poverty, international goodwill; an era that should have demonstrated that Democrats knew how to run the country and left the GOP badly marginalized -- is successfully tarnished.
So the vice president seeks the presidency. He runs a soggy campaign, true. But again, it's beyond dispute that the majority of Americans who go to the polls intend for him to be the president. Yet he loses -- according to the rules, at least. But somehow the experience of the previous eight years has left us with the distinct feeling that, had the situation been reversed, other rules would have been found to ensure the same result. We are admonished to "get over it" by people we know would not have gotten over it if things had gone the other way.
The Republican takes over. For eight months, he convinces precious few who didn't vote for him that he's the man for the job. But then unprecedented tragedy occurs. Americans, the vast majority of liberals included, rally around their country; by and large we support War No. 1. We have serious reservations about War No. 2. But by now something more disturbing than a mere policy dispute has occurred. By now, simply asking questions, or refusing to accept the government's assertions at face value, is denounced as something tantamount to treason. We find this, um, troubling: Open debate and vigorous dissent, we were raised to believe, were once considered the quintessential American values. Now, they are taken as prima facie evidence of anti-Americanism. (We note also how ardently the other side seemed to believe in vigorous dissent when its members were the dissenters.) In Georgia, a man (and sitting senator) who sacrificed his body for his country is labeled unpatriotic. The president has it well within his power, by simply uttering a few morally forceful sentences, to put an end to this madness. But the demonization of the other side is what keeps him afloat politically, and he refuses to do so -- and, in the Georgia instance, goes so far as to implicitly play along.

Even if that description is 100% accurate (and I'm resisting the temptation to unload on the accusation of "asking questions=treason"), this is the best illustration of David Brooks' diagnosis of self-defeating rage. Ask the mischievous Mark Steyn:

They’ve let post-impeachment, post-chad-dangling bitterness unhinge them to the point where, given a choice between investigating the intelligence lapses that led to 9/11 and the intelligence lapses that led to a victorious war in Iraq, they stampede for the latter. Iraq was a brilliant campaign fought with minimal casualties, 11 September was a humiliating failure by government to fulfill its primary role of national defence. But Democrats who complained that Bush was too slow to act on doubtful intelligence re 9/11 now profess to be horrified that he was too quick to act on doubtful intelligence re Iraq. This is not a serious party.

Or ask the judicious John Judis, whose belief in an emerging Democratic majority does not blind him to the fact that Howard Dean's rage-based campaign is likely to end disastrously for the Democrats (ad viewing required):

Even if the United States remains bogged down in Iraq, and even if popular doubts about the invasion and occupation grow, Americans are still likely to credit Bush with trying to wage a vigorous war against terror. And they will consider voting for a Democratic candidate only if they believe he can do likewise. The Republicans will argue that an antiwar candidate like Dean who has no foreign policy experience is ill-equipped to protect the country from attack. And a lot of people will believe those charges. At the least, a candidate like Dean will have to spend a vital part of his campaign defending his credentials on homeland security and the war against terror rather than attacking Bush's economic program. Think of Massachusetts Gov. Michael Dukakis (who, unlike Dean, served in the armed forces) unsuccessfully defending his foreign policy credentials against Bush's father in 1988.
...To put it in regional terms: Dean, a culturally libertarian New Englander who opposed the war, could virtually forget about winning any Southern or border states. Southerners are willing to support a Southern Democrat like Clinton with whom they can identify, but they will not vote for a Dukakis or Dean. Dean would not simply get trounced in the South: His candidacy would allow Bush to take the entire South for granted and move all his resources into states like Michigan and Pennsylvania that the Democrats have to win. In the end, Dean would be lucky to hold on to Vermont, Massachusetts, Rhode Island, New York, D.C., Maryland, Illinois, Minnesota, California, Oregon, and Washington.

A final reason for why the "Bush lied" theme is mostly based on resentment and desire for revenge. Ronald Reagan made all sorts of weird economic claims (held in at least as much contempt by the professional economic set as Bush's claims) and other, shall we say, reality-challeneged statements. (I like the guy, but it's true.) While Democrats savaged him on all sorts of grounds, I don't recall them calling him a "liar" 250,000 times a day. That doesn't make it right to misuse/mangle/ignore facts, but you do get tired of seeing it called "unprecedented" on the NYT op-ed page twice a week when it's simply untrue. (A lie?)
After all, to quote Steyn again:

In 1998, when Bill Clinton launched mid-Monica cruise-missile attacks on Afghanistan and the Sudan, he hit a Khartoum aspirin factory and missed Osama bin Laden. The claims that the aspirin factory was producing nerve gas and was an al-Qa’eda front proved to be untrue. Does that mean Clinton lied to us?

4) Finally, the notorious site run by Al Gore's old roomate, the Daily Howler, has twice defended Bush against the charges of Niger-based lies (here and here; with links from Instapundit.).
After those posts, I will take their criticisms of George Bush and the media much more seriously.