Depends on the source. Wash Post had a story the other day that Florida early voting (walking into polling places early) was running heavily democratic. Some countered that by saying that Republicans were more likely to vote absentee (via the mail) so they wouldn't been seen in as high a number at the polls.
Zogby announced in his daily poll the middle of last week that those that had already voted in the first few days of early voting was running 50-48 for Bush, but the sample size would necessarily be so small (400 national interviews per day...take a guess on how many had actually voted early...perhaps 5% or 10%???) that the data was suspect. I haven't heard anything yet from him or anyone else about early voting, but obviously these polsters will get increasing hits in their phone calls.
Probably because more Republicans work for a living than Democrats.