I realize that Apple's PA Semi unit seems to be working on ARM-like processors now, but how hard would it be for them to pop out their own graphics processor?
Nearly impossible with Apple's resources. They have zilch knowhow in GPU design and only a minor share in PowerVR intellectual property (which applies mainly to cellphone graphics chips).
If a huge chip company like Intel can't do it "just like that", Apple can't do it either. And look at how Intel's try at high-end GPUs failed miserably (Larrabee)