It could also be related to the possibility that Apple didn't like the notion of a large portion of their users' voice and text communications passing through an untrusted 3rd party, adding a potentially large security vulnerability that could reflect negatively on them. I'm not saying that's the reason they did it, but simply that it doesn't necessarily demonstrate influence from AT&T, though I'm sure they, along with most other dominant carrier partners around the world, were not too thrilled with the prospect.
Another speculation is that Apple saw Google Voice as a platform threat of the same variety as Flash, where Apple's users would potentially become dependent on a 3rd party platform. Obviously, it would also ease migration from the iPhone. There are several plausible reasons that don't involve AT&T.
Separately, it's funny you should mention Skype and Facetime. The move with the Facetime 'open standard' seems aimed squarely at Skype, a shot across their bow, while Apple empowers them with iOS 4 multitasking support.
This is also interesting in the iPad. It had a mounting spot for a camera in the body, but shipped with it empty. Was the concession made to AT&T, who would offer a cheap data plan in exchange for refraining from giving the platform away to Skype? Was it simply that Apple needed more time to develop other plans in the VoIP voice/video space? Is it a form of coerced obsolescence? My guess is that Apple and AT&T (and other carrier partners) agreed to hold off on the camera until next year's 4G network model, as the wireless network could conceivably support widespread use of videoconferencing, making their offering much more competitive against Skype.
Totally different network architectures. GV is all routed centrally, from my understanding. Apple cares at least to the extent that it doesn't cause PR issues. In any case, my point is merely that we don't know how much pressure AT&T put on Apple over the decision.
