I know you had asked this a long time ago, but here we go!
Yes, GSM is new technology in USA but used all around the world. Think of it this way, this country was wired to work on CDMA (Verizon, sprint, etc) and TDMA (ATT, Cingular, etc) and now some carriers are switching from TDMA to GSM and they have to put up towers for GSM network. Until they replace all TDMA towers with GSM, the latter will be spotty.
CDMA on the other hand is here to stay, and they are switching from Analog (old technology) to Digital (new technology). Same here, untill all towers are converted to digital signals, you will still need phones that work on old technology to function properly.
I hope that is clear.