As Stu Rothenberg notes, it is actually the norm rather than the exception to have the VP named relatively close to the convention. Still, there is cutting it close, and then there is leaving yourself no time at all. Geraldine Ferraro and Al Gore were named 4 days before the opening gavel at the convention, Lloyd Bentsen 6 days, Joe Lieberman 8 days, and John Edwards 20 days ... so this pick will set the modern record for the Democrats, although the Republicans have sometimes waited even longer....All of this points strongly to the known knowns: Joe Biden or Hillary Clinton.
I have to say that the choice of Springfield suggests Clinton to me. It's the whole Team Of Rivals concept reborn. Texting the selection of Hillary at 3 am this Saturday morning would be the coolest campaign gimmick of all time. Except that every indication we have is that Clinton was never seriously considered.