There are two ways GV is a threat to AT&T right now:
Directly: I could use it for ALL my SMS needs from the iPhone. Not as convenient as SMS on the iPhone plans but it works. I can also choose to use GV some of the time, like when my plan is low on SMS messages.
Indirectly: I can use my landline, conveniently, for free outbound long distance, resulting in less use of my iPhone voice minutes. Otherwise I'd be using iPhone minutes to take advantage of free long distance.
The comments about voice quality are moot. This is all about AT&T stifling innovation - which is what they do best. Where have the carriers been with all the features GV offers? Shouldn't their expertise have made it easy to offer these things, even if for a fee?