Looks like the Million Dollar Question has finally been answered: According to the guys at OXM, Yes, the XBox 360 version of UT3 will have mods. So sayeth Mark Rein himself:
Rein confirmed that Epic is talking with Microsoft about getting round the company's reluctance to share user-generated content, and is "cautiously optimistic" that a solution is coming.
He did say that it was unlikely that "user exchanged mods" would be included in the game because it's so close to release, but Epic hope to have a solution at a later date.