Console shooters suffer from the "can't turn around as fast as a PC gamer" problem. That alone is a decent reason to keep the console players away from the PC players. At least in theory.

Honestly I believe that Ishould be able to play my friend using his 360 to play UT3 with my PS3 to play it, while another friend uses his PC. Hopefully either this is implemented eventually or people might start finding ways to mod UT3 to do so.