For the time being its (UC) xbox only. That game is being made just for console gaming, with tweaks to make it more comfortable for the console gamer (including weapons tweaks), so it would be lame if it came to PC as is. Besides I hear U2 will have a multiplayer mode that is ATLEAST as good as UT. I would like to know if it has a specialised weapon set, cos some types of weps are better for single player gaming than multiplayer deathmatching.
Also, they can't and won't release (because that would make it UT for the PS2 all over again) UC until Microsoft sorts out how they are going to run their online service and actually get it running. The release is set to coincide with the launch of Xbox's online service.