Alec has a perspective, and it's one that I share. I really don't think AT&T cares about GoogleVoice as it drives airtime any more than they care about Yahoo being on their handsets, and Yahoo having an IM identity core via Messenger that also works on their handsets. As a matter of fact, airtime consumption and driving minutes used on their network is how they make money so if GoogleVoice makes calling easier why would AT&T care.
Alec point about number portability and customer's leaving--BFD. People don't leave a carrier because of anything but three factors:
3) Customer Service experience
Features...for the most part people still use their mobile phone for three things:
1) Make calls
2) Receive calls
3) Receive voice mail
Oh, and outside North America add in Text/SMS for adults, while in the USA SMS has clout with kids and teens.
I still go by what I said a few days ago. This back alley scuffle is all about brinksmanship, something Om Malik and I are in alignment on (you can tell as he used a Poker hand photo and I called it "table stakes" in our respective posts.)