Much of the world uses 900MHz and 1800MHz for GSM. If there's any CDMA, it's on 450MHz. The 3G consortium decided to use 2100MHz for the downlink and 1900MHz for the uplink for UMTS because those aren't used except in North America, where 2100MHz is used for satellite communication and 1900MHz is the PCS band. The FCC cleared out part of the 2100MHz band, but the 1900MHz band is already in use, so they freed up part of the 1700MHz band to use instead of it.
Cingular (actually, AT&T Wireless) jumped the gun and decided not to wait for AWS, so they took some of their 1900MHz spectrum, got rid of some GSM carriers, and put UMTS up. Cingular is doing more of that now, and they will eventually put UMTS on the 850MHz band as well. AWS hasn't yet been deployed for public use by any carrier, and there are no phones that support it yet.