This would be okay (grudgingly), given precisely two conditions: If everybody on AT&T still had unlimited data (they don't) or if AT&T only applied the throttling to the heaviest users who still have unlimited data. In other words, if AT&T started slowing down the connections of users who don't have unlimited data—the ones who pay for every byte they download—that would be total bullshit, since, well, they're paying for all of it. Effectively, they'd be getting screwed on both ends.
AT&T won't say a word about it, so it's hard to say exactly how they'd apply the throttling. Verizon's reps haven't gotten back to me as to whether their throttling will also apply to heavy users who don't have unlimited data plans, now that they've switched to tiered data plans too. Obviously, it'd be crappy on their part to hit those users twice, too. (Verizon's supposedly trying to limit 3G FaceTime to people who have limited data plans, to keep the network from exploding, and obviously to make more money.)
If this happens, and AT&T does start throttling its heavy users, it'll leave Sprint as the only major carrier that doesn't implement throttling at all—not to mention it's the only carrier with truly unlimited data. And bless them for that. [9to5 Mac]