Why bother with an Apple OS? Keep the iPhone in the iPhone. Instead of using Apple's icons and having to pay exorbitantly fr it, you may as well just have Sony come up with a similar home screen. Besides, if it ran OS X, then it wouldn't be a PSP, it'd just be an iPhone with a PSP body.
I like the idea of making it a sliding phone and everything, but make it unique. Don't just knockoff the iPhone.