Something to keep in mind:
John McCain isn't getting the GOP nomination next month because the Republican primaries offered voters an embarrassment of riches, and McCain stood out as superlatively attractive among a field of widely popular candidates.
McCain is getting the nomination because the Republican voters, especially the base, didn't like him much, but on balance they liked the other candidates even less. He won simply by surviving a long, unhappy process of attrition.
One consequence of this is that, while McCain's support among registered voters is not that far behind Obama's in a straight head-count, McCain voters are much less enthusiastic about their candidate than Obama voters are about theirs.
So when McCain looks among the other grumpy old men from those GOP debates last winter for a possible running mate--Romney is most often mentioned lately, although his conspicuous wealth could kill his chances in the wake of Housegate--he's reaching into a barrel of names already proven to be even less popular than he is, which is no small accomplishment.
Predictions by Republican tools that Obama will leave the convention next week with a 15-point "bounce" are crude attempts to work the expectations game--when he doesn't hit that improbable number they'll point to it as evidence that his candidacy is fizzling. But the 2008 Democratic convention--which so far seems to be the most shrewdly-orchestrated example of its kind in many a year--will undoubtedly give Obama some bounce in the polls, and the choice of Biden as running mate will undoubtedly contribute to that.
Who can McCain pick as a running mate that would give his campaign a post-convention buzz? For all his lingering "maverick" nonsense, he certainly can't pick a social moderate (although that would certainly create buzz, at the very least). Anti-war Democrats aren't happy with Biden's vote for the Iraq War, but only the most hard-core doves will stay home on election day, or vote for a third party, because of it. But if McCain named someone who wasn't, for example, a solid opponent of reproductive choice, his base would chase him up a tree and set fire to it. McCain's going to have enough trouble reining in the culture-wars people at the convention as it is.
Choosing a running mate for his heavyweight foreign policy/national security credentials wouldn't be much help, since that's supposed to be McCain's strong suit.
He could pick someone from a big electoral vote-count state, but those math-based choices rarely light a fire under voters.
He can't pick someone too much younger than him. And so on.
So how will McCain's veep choice produce a post-convention bounce? Here's my prediction: It really won't.
McCain's choice, whoever it is, will mostly fail to catch fire with the public and the Republican convention will be mostly reduced to a week of "noun, verb, POW," with a nightly dose of no-quarter attacks on Obama and Biden, as the 2004 GOP convention handed Kerry (suggesting that any "bounce" McCain does get might be from pulling down Obama's numbers as much as lifting his own).
Don't expect much trumpeting of the VP pick, and don't expect too much on the substantive issues either.
And a week after it's over, don't expect many people to remember it happened.