UT3 was never exclusive to the PS3. This title was also slated for the PC. If a title comes out on the PC it will eventually make it to the XBOX360 too.
Mark Rein the head of Epic never said it won't come on the xbox360, he just wanted to have a confirmation from microsoft that they'll allow mods on their console.
Looking forward to the PC or PS3 version.
Btw it's UT3 not UT2k3.
Sony has been losing a lot of third party exclusives lately. Unreal, GTA, Assassins Creed, SquareSoft (still not the single player Final Fantasy, only the online). They are even losing some PS3 exclusives to the PS2 (God fo War 2).
Exclusives are still what help drive console sales. The next big thing comes along and people upgrade when they have to.