I agree with Nate, the Obama campaign should call the McCain people on this:
In preparing for a debate, you are often preparing nearly as much for the "judge" or moderator as for the opponent. Both campaigns probably have a pretty good idea of what types of questions she is likely to ask, how he is likely to ask them. Palin and Biden have undoubtedly watched videotapes of Ifill moderating the 2004 debate between John Edwards and Dick Cheney.
By changing the moderator, you're throwing everyone a curveball, and catering to the candidate who is better able to adapt on the fly. Which, most likely, is not going to be Sarah Palin.
I highly doubt that the McCain folks actually want Ifill to withdraw. They just want another stick to beat the "media bias" drum. Call their bluff. I love Gwen Ifill, and I frankly don't think that they're going to get anyone fairer. But they should call them on this. Let nothing stand in the way of Sarah being Sarah.