My Xperia X10 seems to be experiencing issues. I have tested the data speed of my phone with the app "Speed Test" from the Android Market and got some very poor results. I have been monitoring these tests, and no matter where I go, the download speeds on 3G never break above 500 kbps, and the uploads speeds never break 200 kbps. I have friends that own the same exact device, as well as other members in other forums and they get download speeds of 1000 kbps over AT&T 3G. There is definitely something going on.
Did you do the OS upgrade? The only way to get the faster speeds is the flash the phone. Mine is running generic 2.1 and using HSPA (H). UPLOAD and download speeds vary due to location and other variables.