Nate Silver makes one last pitch:

If Obama picks Clinton, the Republicans are likely to overplay their hand. One thing that Obama has not really been able to do is to generate some organic level of backlash when he is attacked. This is separate and distinct from the notion of "fighting back"; it is voters stepping in and refereeing the match themselves. Voters recognize that McCain has gone negative but they aren't really punishing him for it -- his favorables haven't moved at all. Why not? 

I think it has to do with the nature of Obama: he is new, he is confident to the point of being arrogant, and up until recently, he has been leading. To the extent there is any genius in the "celebrity" line of attack, it's that nobody feels much sympathy when celebrities are made fun of (well, except for this guy); it is a sort of sport to try and pierce their bubble.

With Clinton, on the other hand, voters naturally want to come to her defense -- and overzealous attempts to whip the Republican base into a frenzy will be counteracted with outrage from significant numbers of older and working-class women.