For Obama, one of the pleasing effects of having Joe Biden on the ticket is how hard it makes John McCain's veep choice.
Until the Biden announcement, the McCain campaign was giving heavy hints that it would be either Mitt Romney or Tim Pawlenty. But suddenly they both look problematic. Romney contrasts poorly with Biden on three counts. First, he was born with a silver spoon in his mouth, and is now the owner of as many homes as John McCain; Biden is a working class guy who's now a middle class guy. Second, he's got no national security credentials and whilst he's a good study he won't look authoritative next to Biden. Third, his biggest political problem is that he comes across as a robo-pol with no heart or soul. Biden's emotional directness will play well against Romney in the debates. As for Pawlenty, he's got even less national security cred than Romney. So now apparently McCain is considering Meg Whitman, former CEO of eBay, but she's untried and untested as a politician and might be even more likely to flunk foreign policy questions in the debate.
Of course, the guy McCain would love to pick is Joe Lieberman, who has national security experience and would be a good all-round match for Biden. But the pro-lifers would be furious. McCain might go ahead and choose him anyway but risk having his pick overshadowed by stories about dissent.
All in all, it's quite a spot for McCain to be in at this stage, the week before his convention.