NO CHANCE. DON'T WAIT. NOT COMING.
Skype on the go = you use the phone less (hint: ATT gets mad)
Well, let's split this into 2 parts:
VOIP over Wi-Fi = AT&T very happy. You still have to pay for a minimum amount of their minutes, and you're using their network way less. You're a paying customer who's voluntarily using their service LESS. Good news for them.
VOIP over 3G = AT&T indifferent. Yes, you're using their cell towers and 'net connection, but you're not using their phone network computers to make connections. So it's kind of a draw. You're using a little bit less of their infrastructure, but still paying for minutes. This isn't as great as VOIP on wi-fi for them, but it's not really a loss.
IF you could buy an AT&T plan with 0 minutes, they might get mad when people sign up for that. But they don't, and you can't, so you're still paying them for a certain amount of minutes no matter what.
Anything that means you use AT&T's computers less (yet you still pay them for minutes) is good news to AT&T.