Not so fast. "tandard GSM global frequencies" are the GSM 900 MHz and DCS 1800 MHz bands, both used extensively outside of North America, neither used in North America (at least, not in the three countries that matter). Now, quad band GSM -- Cellular 850 MHz, GSM 900 MHz, DCS 1800 MHz, PCS 1900 MHz -- has become ubiquitous. Is that what you are calling "standard GSM global frequencies"?
Regardless, the lone standard W-CDMA global frequency is the UMTS 2100+1900 MHz band, which is not in use in North America. (The Eurasians like to screw with Americans, even though or, perhaps, because the US is still the single most important market in the world.) AT&T requires W-CDMA in both Cellular 850 MHz and PCS 1900 MHz bands, both of which may be common on many international handsets but are by no means given.
AJ