Well blow me,
I think AT&T have a point here. But lets not be too hasty here, lets not cut off all the adult chat lines for all those lonely guys (and girls?) out there. The problem here is that if the commission favours AT&T, they can do three things 1) tell Google to get back in line or 2) Change the rules to allow blocking of certain call types or 3) Say Google Voice is not applicable to those rules as a non-traditional service.
The latter option could open a whole bag of worms with the carriers potentially deciding that they don't like certain lines and there's SFA you can do about it. Maybe that is an over generalisation, but it would need to be protected from financial and political abuse.