The good news for Obama, however, is that it gave him a chance to tweak Hillary yet again about Iraq. I don't know for sure if that's a winning strategy, given that his forward strategy for withdrawal isn't very different from HRC's, but it's a helluva lot better than Social Security. If he wants to disinguish himself more sharply from Hillary, this is the place to do it.
This is all true, but it's worth going non-meta here. Hillary Clinton's past support of invading Iraq doesn't really tell us anything about her forward-looking Iraq policy. And it's true that both candidates have left enough vagueness in their forward-looking Iraq policies that it's hard to say if they'd do things any differently. But past conduct vis-a-vis Iraq isn't a predictor of forward-looking Iraq policy but it does offer a glimpse at various other issues.
The thing that I feel people who want to discount the Iraq issue or write it off as some kind of teenage foible are missing is that the Iraq debate had actual content about the appropriate shape of American foreign policy. In particular, after 9/11 a lot of people -- Matt Yglesias, Hillary Clinton, Kevin Drum, George W. Bush, John Edwards -- decided that it was important for the United States to become more willing to engage in preventive war to halt the proliferation of nuclear weapons. Obviously, I'm not going to stand here and tell you that that was an unforgivable mistake, since I made it myself. But since I've decided that that was a mistake -- not just Iraq, but the change of heart about preventive war that led me to support Iraq -- I'd like to find a candidate who didn't make that mistake (Obama) or who like me now thinks it was a mistake. Hillary Clinton, as best one can tell from her record, her public statements, and the views of people associated with her campaign, doesn't think that was a mistake.