For some reason I had a hunch that it would be Biden. Maybe it's just that media discussion of Biden has seemed stronger and more consistent than other choices lately.
I find the Biden pick pretty strange. On the "soft qualities" side of the coin (by which I mean everything other than their policy positions, which are largely similar between the two) they are total opposites. Biden is an old white dude with significant experience on the national stage who seems to be able to relate to the working class. If anything, I think the pick just highlights Obama's inexperience and appearance of being an out of touch elitist.