Quick question: AT&T Wireless SIM unlocking.

I have heard that AT&T Wireless is particularly uncooperative about unlocking handsets. For that reason, I'm considering T-Mobile instead. (I want a quad-band GSM phone, and I think those two carriers are my options around here, here being Connecticut.)

T-Mobile's policy is that they'll give you an unlock code after 90 days, which for my purposes is sufficient.

Am I wrong about this? AT&T Wireless's site is not being particularly helpful here.

Thanks, all.