It's entirely speculative, but it occurs to me that insofar as the contest to be John McCain's veep looks at the moment to be between Mitt Romney and Tim Pawlenty, the Biden pick probably elevates the former's chances at the expense of the latter's. Unlike Tim Kaine, Kathleen Sebelius, or even Evan Bayh, Biden has genuine political stature and authority, especially on foreign policy, and he's an extremely talented debater. I suspect the McCain camp would be fairly nervous about balancing him out with a youngish Midwestern governor new to the national stage. Romney, by contrast, is a more proven commodity who's already had the warmup of several presidential debates, is polished on TV (sometimes to a fault), and is obviously willing to say or do pretty much anything McCain asks of him. (For the record, I think Romney is a much worse pick than Pawlenty, Biden notwithstanding.)
I suppose the counterargument would be that Biden's working-class roots and poorest-Senator background make it harder for McCain to put a gazillionaire like Romney on the ticket (how many houses do you think he owns?), but I doubt this would outweigh the (perceived) stature gap. Of course, if you believe Marc Halperin's sources, Romney had it sewn up even before the Biden pick.
--Christopher Orr