All media outlets are reporting it's Biden, which suggests the Obama campaign is ready to take the foreign policy fight to McCain. There'll be more to say soon enough, recapping Biden's effort to disrupt the war in Iraq through the Biden-Lugar resolution (though when that failed, Biden voted for the war), noting that his son is going to Iraq in October, mentioning that he authored the Violence Against Women Act, and playing up the fact that he chairs the Foreign Relations Committee. But if you really want to know why a Biden choice makes sense, watch this video, particularly the facial expression at its end:
For longer thoughts on the subject of Biden, read this piece.