If this works as advertised this will be my #1 app.
I've never had AT&T, but I've heard the coverage suffers in comparison with Verizon and Sprint. But if this baby is for real this could be a non issue. The question is whether you can initiate a call while on wireless not just transfer from GSM? Also this will be an Apple approved app.
Check out the video.
iCall - Free iPhone VoIP