Right, Apple has been designing their own integrated mobile GPUs for a while now using Imagination IP, and they are great for their laptops/tablets and low to mid end desktops. But going from an integrated mobile GPU to a full blown discrete GPU is not a trivial task. They could choose to scale up their mobile GPU designs to something that performs at the level of a high end discrete GPU. Or, because good high end discrete GPUs that they are already using are available, and this only affects one or two machines at the high end that they sell at low volume, they might just stick with AMD.