There won't be a switch away from nVidia. Here's why:
nVidia makes the chipsets for Apple computers right now. If Apple starts using ATi GPUs, nVidia won't like that nor will Intel. So then the only choices are to go back to nVidia or to use the crappy Intel graphics cards.
It just won't happen.
So? Intel only makes iGPUs so they wont care. Apple wont care if nVidia does not like it. They could easily just include an ATi HD4350 in the low end segment and use Intel Chipsets which are a lot better IMO.