Unless you're living under a rock you would know that the phone doesn't just have tmobile frequency support, but also At&t 3G bands. According to their own site, SE insists that the only USA released X10 will support At&t 3G bands, though they are pretty notorious for never updating information on their own site.
It's like you'll be able to buy an unlocked version of the phone and use it with Tmobile 3G, but I find it difficult to believe that T-Mobile would subsidize the phone along with the large assortment of Android phones it already has in the pipe-line.
On the other hand, a few accounts with no real evidence to back them up have spoken of the X10 in fact being one of the 2 android phone At&t is releasing in upcoming months. Right now we know of Dell Mini 3, Motorola Backflip, and HTC phone (likely the Bravo).
Several corporate employees have discussed recent meetings at the HQ discussing upcoming changes to their networks, new phones etc. All people that have mentioned the meeting have said they played with 2 of the upcoming Android phones that At&t will be carrying. Included in those 2 was the Xperia X10, and backflip. A few even mentioned possible X10 variation for different price points. i.e. dumbed down OS etc for cheaper etc.