I have a theory that Democrats dominate among early voters because early voters skew towards first-time voters, as many people who have a history of voting would vote specifically on election day as a force of habit. What we will need to look at to see if Obama is going to have a big advantage is not the exit polls of early voters; but, how many early voters there are in 2008 versus 2004.
Actually not, as Nate Silver mentioned at his site:
According to a study by Kate Kenski at the University of Arizona, early voters leaned Republican in both 2000 and 2004; with Bush earning 62.2 percent of their votes against Al Gore, and 60.4 percent against John Kerry. In the past, early voters have also tended to be older than the voting population as a whole and more male than the population as a whole, factors which would seem to cut against Obama or most other Democrats.