The prospect of PC users going toe-to-toe with PS3 gamers in a game of Unreal Tournament III was always a possibility, but never certain. So your knickers shouldn't get too twisted at the news the game won't support the feature when it launches. Mark Rein (posted from an IRC transcript, if you're wondering about his statement writing):
The biggest challenge in doing cross platform play is syncronising the builds such that the build on the PC is 100% compatible with the build on the PS3. Unfortunately (or fortunately) when you games on console you have a lengthy certification process to go through each time you release anything new...
We looked at how this would impact our ability to respond quickly to things happening at internet speed on the PC side of the equation and realised that would not be in the best interests of our very loyal PC user base because we would constantly be holding on to updates to wait until they passed cert on the console platform.
PC users, Rein and Epic got your back. PS3 owners hoping for cross-platform play...well, he goes on to say that they might be able to institute the feature later on, but really, I wouldn't bet on it. It's a bet you'd lose money on. And nobody likes losing money on stupid bets.