Actually, word on the street is that either way, you can probably expect a new AG in January. Seems Bush and Ashcroft have grown apart somewhat over the past 4 years. That could be baseless rumor, though.
I have heard a few people say that they are growing apart, generally as part of an advertisement that Bush's next term would be more secular. But evidence to back this up seems to be lacking (I've seen none, anyway). By all outward appearances, Bush is as or more committed to guiding a socially conservative policy with faith as he ever was- and Ashcroft is a great tool to this end. So I don't even really understand why he would want a new AG in the first place.
I suppose that Ashcroft could be made into a pre-election ballast dump. But were that the intention, surely they'd have made it known at this point in the campaign.