I know the X1a is the one for US and the X1i is for the UK. I've read that there will be 2 distinct differences.
1. the US version can only record video at QVGA at a slower frame rate. UK version has VGA at 30 FPS.
2. the Xperia has 2 versions, X1a and X1i. Both have tri-band HSPA. The US version (X1a) should have the 850/1900/2100 MHz. And the international version (X1i) should have the 900/1900/2100 Mhz. Both versions are quad band EDGE. In the US, we have 850 and 1900, under AT&T. What this means (and i could be mistaken) is you'd get normal phone and EDGE reception. BUT, when you use the fast internet, 3G, you'd only be supported in areas where there are 1900Mhz. This might not be an issue depending on where you live.
anyone know if there are any other differences?
(repost from KevVo)
Q: UK and US Version - inxperia forums