Nvidia and Apple already have a working relationship. Unless something tragic happens, I don't see apple jumping to the other camp.
What are you talking about? Apple has consistently used both ATI and nVidia GPUs in both their laptop and desktop computers for years. Right now, Apple's iMacs come with a card from the HD 4xxx series, with an HD 4850 at the high end, and the Mac Pros offer either the HD 4850 or GeForce GT 120 (the OEM 9500 GT, pretty much). Macbook Pros come with either a GeForce 9400M or 9600M. So their mobile line uses nVidia cards, while the iMac line uses ATI cards, and the Mac Pros offer you a choice of either. Looks like a pretty even split to me (except for the fact that the available ATI cards are considerably better than the nVidia cards offered...).
In any event, Apple hardly has an exclusive relationship with nVidia.