Epic frontman Mark Rein spoke to some Unreal community members via IRC, talking about various aspects of the game, including cross platform play between the PC and PS3 versions. Sadly, he revealed that it was simply too complicated getting PC and PS3 versions to play together. Rein said:

The biggest challenge in doing cross platform play is synchronizing the builds such that the build on the PC is 100% compatible with the build on the PS3.

Unfortunately (or fortunately) when you [develop] games on console you have a lengthy certification process to go through each time you release anything new.

It’s as simple as that. One can’t update console games as easily as releasing a PC patch, so the challenge is very clear. On the other hand, as Rein mentions, console patches are generally of better quality because of the very same certification process. There is no release date set for UT3, only that the “target” is November. Xbox 360 will be getting the game early next year.