According to SE’s Xperia X10‘s product page, the device supports the following radio bands:
Interestingly those are the bands supported by AT&T in the US. THat’s just a supposition though so we will have to wait and see when the final US version of the product is announced in Q1 2010.
Sony Ericsson ‘s official blog has also uploaded two videos of the device in action:
The Phone Dialer: