For the record, I think early voting is a pretty lousy idea, at least when it starts more than a few days out. Campaigns are intricately-timed affairs--when run well, they have beginnings, middles, and ends, almost like a good narrative--and candidates can't possibly execute them effectively if they're ending at different times for different voters.
Unfortunately, several states now let people vote weeks before Election Day. As Amy Sullivan (my wife) notes in her recent piece on the subject, Virginia allowed some people to start voting last Friday, and half a dozen states kick off early voting next week. So a non-trivial portion of the electorate could end up casting a vote before there's a single presidential debate. It's ludicrous.
Having said that, I can see this preposterously early voting benefiting Obama. For one thing, he gets to start locking in votes while the financial crisis is topic A and has voters fleeing the GOP. Second, he gets to lock in votes before a possible October surprise--whether it's a national security crisis ginned up by the White House or some below-the-belt ad from McCain (probably Wright or Ayers related). Third, mechanical changes like this tend to favor the better-organized campaign, since they're in a better position to exploit any new opportunities. That's almost certainly the Obama campaign in this case. I'd guess Obama will do a better job identifying potential early voters and getting them to the polls,* which, given the extremely long early-voting window, could result in a significant advantage.
*More precisely, I'd guess Obama will have a bigger organizational advantage over McCain in early voting than he will on Election Day, though I'd expect him to have an advantage in both cases.
Update: See this useful Tom Edsall piece for more on the Obama campaign's sophisticated voter ID efforts.
Update II: Frequent commenter virginiacentrist raises a great point:
Early voters are usually just base voters from both sides. It's not going to effect anything either way. All it does is makes it easier for people to vote. It also makes it easier to get your low-turnout base voters to the polls (because they have a whole month to do it). Persuadable voters don't vote early.
That makes intuitive sense. On the other hand, I'm sure there's some number of people for whom the sheer hassle of lining up on Election Day makes early voting attractive even if they're still persuadable in principle. And you obviously pick up more of those people as you expand early voting.
--Noam Scheiber