Engadget Mobile received word from a trusted source of theirs that AT&T will offer the Sony Ericsson Xperia X10 "in fiscal week 30 of 2010, which represents the tail end of July and the beginning of August." It is very possible the date will get pushed around and/or the phone gets dropped before release.
AndroidCentral reminds us that Sony Ericcson doesn't have any smartphones available on a US carrier, so if this deal goes through, it should be good news for Sony Ericsson. In support of this report, Phandroid mentions of the fact that there is already a version of the X10 supporting AT&T bands.
Now we just have to hope that AT&T doesn't limit anything on the phone if it does indeed release and that it receives an update from Android 1.6 to Android 2.1 before it releases to make it more competitive with the other top-of-the-line phones on the market now.
Image from Phandroid