Yup, even though the specs page lists support for UMTS/HSPA 800/850/900/1700/1900/2100, Sony Ericsson is going to be localising the radios. From the source:
UMTS HSPA 800/850/1900 only available for phones sold in Americas (except Brazil) and Australia. UMTS HSPA 900/1700 not available for phones sold in Americas. UMTS HSPA 1900/2100 available worldwide.
So that means you'll still be able to find an X10 that works on T-Mobile, just not in this country. You'll still be able to import if you want to use your expensive handset with T-Mo's cheap Even More Plus plans. But if you want to have that huge $US880 cost subsidised at all, looks like you'll be going with AT&T. [Phandroid via Engadget]