I'm not going to try to claim that Barack Obama hasn't ever changed his stance on Iraq, but this doesn't seem to be such a flip:
From last year:
Democratic presidential hopeful Barack Obama said Thursday the United States cannot use its military to solve humanitarian problems and that preventing a potential genocide in Iraq isn't a good enough reason to keep US forces there," the AP reported on July 20, 2007...
From Tuesday night's debate:
In such cases, answered Obama, "we have moral issues at stake." Of course the United States must act to stop genocide, he said. "When genocide is happening, when ethnic cleansing is happening . . . and we stand idly by, that diminishes us."
Sorry, but I don't see any inconsistency here. He doesn't think preventing a potential genocide is a good enough reason for a military presence in another nation. He does think stopping one in progress gives a moral imperative to stop it. To make this work, he'll have to provide arguments for why the first case is merely potential and not a serious enough risk to worry about removing troops completely from the situation, or he'll have to argue that it's never ok to intervene until after the fact. I don't think either argument is easy to make. I'd need to see more than I've seen to convince me, anyway. I also don't know of any instance when he's tried to make that case. But it's not necessarily an inconsistency or a change in view.
It's perfectly fine to point out when politicians change their views in order to get them to explain the change (but don't assume a change is a sign of flipping for mere political reasons if there's a plausible explanation for that change in views, as there sometimes is). It's also fine to ask them to explain how two statements fit together if they don't think they've changed their views. I don't think Obama's statements on gun control fit together at all, despite his claim not to have changed his views. I also don't think his explanations of his past connections and votes are consistent with each other or with the past. But his critics need to be a little better at restricting themselves to genuine examples of conflicting statements, or even the legitimate questions about his honesty, revisionism, and political expediency are going to be seen as mere political plays without substance.
It's counter-productive to use flimsy reasoning against a candidate, because those inclined to give the benefit of the doubt (and undecided voters probably are) aren't going to be moved by the real criticisms if they constantly see bad ones. The mainstream media and lefty blogs have now ruined their chances at any legitimate criticisms of Sarah Palin move the conservative base, because they can't trust anything anyone says against her. The fact that a political opponent who has been vocally against her has led the investigation of the firinng of Mike Wooten is going to lead to immediate distrust of what little criticism of her their report contains (it's mostly critical of her husband). The base will of course have no problem playing this stuff up to no end, but that's not going to move undecided voters very much.