We already know that Apple has bought into nVidia's fire sale.Do I really need to go over why? In any case, nVidia made a card that gets hot enough to cook eggs and boil water by idling at 90*C, it consumes 300W of power and its MSRP is at $499, but in reality it sells for more. That's just 1 card.
All of that for a card that beats the ATI Radeon HD 5870 by 5-7%, card that consumes less than 190W, idles at half of the Fermi and costs $100 less. Also, ATI cards do OpenCL (version 1.0) but they do.
Now, who you going to choose?