Michael McDonald, an Associate Professor of Political Science at George Mason University and one of the nation's top experts on voter turnout, has created a web page to keep track of early voting data provided by states. Important Caveat: This is a toy! Early voting has barely begun. Do not take the data too seriously!
Not all states report data by party. Right now, Professor McDonald has partisan breakdown data on early voting for Iowa, Maine, North Carolina. Note that these are not actual votes - merely the registration data for those casting early ballots. Democrats may vote for Republicans, and vice versa.
In Iowa, the breakdown of voter registration for ballots returned so far this year is 42.1% Democratic, 28.9% Republican, and 28.9% Independent. In 2008, final early voting registration in Iowa broke 46.9% Democratic, 28.9% Republican, and 24.2% other. Those numbers are for ballots returned. According to the Chicago Tribune, the Democrats also have a narrower margin this year in ballots requested by their registered voters than they have had in past in early voting.
In Maine, the breakdown on ballots returned so far this year, by party registration, is 36.3% Democratic, 37.6% Republican, 24.5% Independent, and 1.5% Green. In 2008, the final count on early voting, by registration, was 41.1% Democratic to 27.7% Republican and 31.2% other.
In North Carolina, the breakdown on ballots returned by party registration so far is 42.2% Democratic, 40.2% Republican, and 17.5% Independent. This compares to a 2008 breakdown of 55.9% Democratic, 27.1% Republican, and 17.0% Independent.
There are several other states that ultimately report early voting data by party registration, but they've not yet checked in.
Again, these numbers really are just for fun. I wouldn't put much stock in them as a harbinger of forthcoming Republican success, and if they turn south, I wouldn't worry much, at least not until a lot more votes are in. But they're fun to look at nevertheless.