See Xbox 360 Fanboy for more details.
Rein responded that [UT3] was definitely on the way, and reiterated that finding a way to incorporate user created mods was causing the hold up. "So our job, when we finish the PS3 version, and weve taken a little break," said Rein, "is to sit down with Microsoft and to figure out how we can do Mods on their system."
That's always good to hear. A 360 version has always been on the cards, but exactly how mods will work is another question, due to Microsoft's strictness over what content is available via Live.
Can they play UT3 PC mods???
Mods will specifically have to be written for the 360, and of course you have to work out how to get them onto your 360 too ;)