I am of two minds about this.
Firstly, the news might allow GWB voters to get complacent, and not vote, figuring he's got it made.
I wouldn't worry about that too much. The guesses about which way the early voting is breaking have been all over the map. No one knows who's ahead right now.
Actually they do know. Ed was saying on one of the Radio shows (Hannity I think) that the early voting has to provide the persons info...so both parties can see who is voting. So they know exactly who is voting and based on that they are determining they have more people voting for them rather than against.