That's going to be the question of the year I think.
As it stands, UMTS in the rest of the world is at 2100/1900
Cingular UMTS is 1900/850
T-Mobile U.S.A is rumored to be deploying at 2100/1700.
So theoretically, for a phone to work on all GSM networks with UMTS, it'd be:
GSM 850/900/1800/1900 UMTS/HSDPA 1900/2100, 850/1900, 1700/2100
And a partridge in a pear tree... welcome to carrier lock in
-olly