I don't doubt that they'll be using IBM chips. Besides, even though microsoft wants to keep the costs down, they have the capital to sell these things at a loss just to pick up even more marketshare.
Either way, console games being developed for a high end PPC processor could be a good thing (although aren't the current PlayStation and the Game Cube already using PowerPC chips?), but I don't know if we'll see more Mac games because of it...