Unreal Tournament 3 are unfortunately out of luck. Mark Rein has stated that there will be no PS3 demo before the game's release. If you have a decent PC, however, then you're in luck. The reason given is that Epic isn't a big enough company to work on a game and a demo at the same time.
Rein said it was the same when Microsoft asked Epic for a Gears of War demo. "What we told Microsoft last year - they wanted a demo for Gears of War and we said 'you've got a choice - you can have the demo this year or the game this year'." We know which we'd rather have. The game is a no-brainer for us anyway, we'll be picking this up as soon as it's available.