In an interview with Kikizo, Mark Rein from Epic said they are doing their best to give the PS3 and Xbox 360 version of Unreal Tournament 3 support for mods:
"We're definitely going to try and have the capability of people who make mods using the PC tools to bring them over [to PlayStation 3 or Xbox 360]. We don't want to get too specific about that, as there's still a lot of hoops for us to jump through with the console manufacturers, and they don't like us announcing their stuff."
He also says why UT3 won't come out for the Wii, but I think that's pretty obvious:
Thanks to some pretty advanced technical requirements on the graphics and central processors, the Wii finds itself unable to run UE3 in its current form. That might not be the end of the story, though, as Rein explains: "I'm sure one our licensees will, at some point, squeeze it down into the Wii, the way Ubisoft squeezed UE2 into the PSP, for instance."