Epic has been wavering on the whole "PC vs. PS3" cross platform game thing in Unreal Tournament III
for months and it looks like they've finally decided that it's just not technically
feasible right now to do so. The main issue is keeping the versions of the game synchronized between the two platforms (Mark Rein sites the long certification process for console patches as the culprit).
There are cheap shots like "If you wrote bug free code this wouldn't be a problem" but given how complex next gen software is that's a bit harsh. I'd also point out that if it wasn't for people cheating at online games this wouldn't be a necessity but that should be obvious to most of you.