Not 100% sure about at&t, but the GSM network I currently service uses TDOA, or automated differential time of arrival at 4 or 5 sites to determine location. Accuracy to 3 m or better, similar to GPS. Not sure if there's a technical reason that you can't use GPS over the GSM like you can on CDMA though. Considering that at&t handsets can be configured for the network I'm at I'd guess they might also use TDOA in which case GPS doesn't matter.

Anyone else know for sure how at&t does 911 location?