850/2100Mhz is the prevalent frequency in use.
Some carriers have announced they are doing WCMA at 900 including VF in Australia (being built by Ericsson at the moment).
This means that most handsets have 850 as the low frequency. The majority of chipsets available for the past 2 years are US made so 850/2100 is the prevalent combination. 900MHz chipsets have been available for about 6 months and handset vendors have them road mapped for Q1 2009.
There is some talk about a Quad-Band GSM handset with TRI-Band WCDMA but nothing solid yet.