There's really never been a question of IF 
Unreal Tournament 3 was coming to the Xbox 360 but a matter of WHEN the game was going to appear on the Xbox 360.  MAXCONSOLE has 
found a German magazine that is stating the game will ship out in April for the Xbox 360.  That seems about right as it would give the PS3 version a four month window.  Honestly I wouldn't expect it until June/July but that's just my opionion.
Thanks to 
Evil Avatar for the link.