Give Charlie Stross a read on his well authored post on his blog, Charlie's Diary.
My take is simply this. If you have GoogleVoice in calling party pays environment, you can eliminate a large monthly base mobile bill for unlimited minutes, because every call is received back to you after being triggered by the data side of the phone. So put your money into a data plan instead.
By reading this, I'd say the mobile operators now have more to worry about than Apple does with Google Voice. The GooglePhone, as a 3G data device will allow Google to manage the entire calling experience. The only issue will be when the user goes outside of a 3G area.