Biden's foreign policy credentials are solid, but I don't think Obama will take chances with him misspeaking again. (I know that's an old, old story, but I think he's too canny to take risks he doesn't have to.) I don't think Kaine buys him much in the South. Indiana has a Republican governor, so forget Bayh. The Democrats should pick up a solid majority in the Senate, but why would they put themselves one seat further from a cloture vote? If you put her treacherously ignorant vote on the Patriot Act aside, Clinton might actually be the best qualified, but while choosing her would probably draw a lot of support from women, she is still a lightning rod for right-wing hatred. You put Hillary on the ticket, and a lot of yahoos will show up to vote against her that would probably stay home otherwise. Sure, that's a ridiculous attitude to have, but it's there. By picking Sebelius, he gets a partner with a strong personality, who should still be attractive to women, but who does not attract the hatred.
Obama's pattern has been one of canny calculation. He doesn't float trial balloons or tip his hand. He has not been about conventional wisdom from the start. I expect him to do what's not expected, what hasn't been yammered on and on about for the last week.
[Edit 1:07am:] Neeeeeever miiiiiind. Just got the text message, it's Biden. *shrug*