Considering that they just released the new Mac Mini's with the Nvidia 9400M chipset in March and just upgraded the entire line in October, I can't see why they would make any changes in the next quarter. My guess maybe October 2010?
Also, you do understand that the Mac Mini is really there to get people to switch to Apple from their PC's. It isn't designed to be a high end unit. If they did make it a high end, then it would end up competing with the Mac Pro's. Not gonna happen. The Mac Mini will always be several generations behind.
Oh and let's also not forget, that how do you deal with the heat of a discrete video card/high end processor and still keep the unit at its current size? You can't.