The general assumption is that Obama supporters, whoever they may be, are extra energized this election, and thus turn out in higher proportions during early voting than they will on election day. So even though Obama likely has a lead in early votes in Georgia, those who turn up on election day will skew more hevily Republican and McCain will pull off the win.
Why? Not talking about Georgia specifically, but why is the assumption that voters on election day will be less prone to vote for Obama than early voters?
The more voters who turnout early, the less that can show up on election day.... unless of course you're a Republican in Philly and you do both.