After being rumored once upon a time for AT&T, it looks like the Sony Ericsson Xperia X10 will land on the magenta carrier, T-Mobile. The Xperia X10 FCC manual lists the device as having 3G Bands 1, 4, and 8--which are the bands for T-Mobile US AWS 3G. This does keep in line with what we heard recently but knowing the history of Sony Ericsson's Xperia line (and how US carriers never offered the X1), it's still semi-surprising to see actually happen. Though maybe since the Xperia X10 is a 'flagship'-type Android device, T-Mobile saw an opportunity to bolster their weakening Androind lineup (relative to Verizon & Sprint).
If true, the Xperia X10 has infinitely more chance of success in the US with a US carrier attached. Who's interested?