Already out on T-Mobile..
So until next week or whenever the other networks release it, I guess you could say it's exclusive to them!
Orange has said that it has won the exclusive on Sony Ericsson's eagerly anticipated Android-based smartphone, the Xperia X10. That claim came as some surprise to Virgin Media, which announced last week that it will be offering the X10 in April. Sony Ericsson Xperia X10 Sony Ericsson's Xperia X10: if you want the white one …
If the standard of their Windows Mobile UI (Panels) is anything to go by I wouldn't touch this with a barge pole. The first thing you had to do with the X1i was remove the dire Panels interface and put something decent like Mobile Shell, Winterface or TouchFlow 3D on there. And the new version on the (will it ever be released?) X2 doesn't look any better. Prettier but just as slow and prone to hang.
No date confirmed yet, but Sony have announced that there will be an upgrade.
As for whether it's a better bet than a Desire, I'd say it comes down to whether you like the whizzy social networking integration and superior camera of the X10 more than the slightly more vanilla look of the Desire.