att and t-mobile are theo nly gsm carriers in the US.
that would be a misnomer. there are several GSM carriers, at&t and tmobile just get more advertising. Jolt mobile, airlink and boost mobile are GSM carriers as well. .
Goblit for what your talking about that is for CDMA phones with verizon or sprint. The cool thing with GSM is you ahve a little sim card, kind looks like a smaller SD card that holds all of your information and you can swap it in and out of different phones and usually it will work.
I'm going to vouche for Kidan's stuff too, and agree with him, LG for sure, motorola hesitant yes. I dont like LG personally though because they usually have horrible batteries.