According to the IRC chat that featured many members of Epic UT3 team, the UT3 cross platform multiplayer will not happen, at least not at launch.
The rumor that PC users will have a chance to finally battle the PS3 players in Unreal Tournament III has started back at E3. Even then it was just the rumor and it was never confirmed that it could be possible.
Finally Epic's VP Mark Rein has put an end to this rumor. "The biggest challenge in doing cross platform play is synchronizing [sic] the builds such that the build on the PC is 100% compatible with the build on the PS3," said Rein.
Mark Rein even stated that the future might bring the cross platform multiplayer but that he wouldn't bet on it. Sincerely neither would we.