Everything looks up to the standards of a high-end PC. This might be of importance to behind-the-times PC gamers looking for a way to run the game with nice visuals and a solid framerate.
On that note, mouse and keyboard control is supported. I had no problems getting my keyboard recognized, but I had to unplug and plug in the mouse and change the USB port I was using several times before it was recognized, so watch out for that. You can use the keyboard for basic menu navigation, but you can't actually fully navigate all menus with it, which smacks of laziness.
Once you're in the game, however, the mouse and keyboard are fully functional. You can specify whether you'd like to see servers with just controller players, just mouse/keyboard players, or both. Sensitivity and keyboard key mappings (but not mouse button mappings) are fully customizable, as are controller mappings. It accepts input from all devices simultaneously, so you can even use the left side of the controller for analog movement and the mouse for aiming, though it would take some creative remapping.
The game really does feel totally different depending on which configuration you use. Put simply, it feels like a PC game with the mouse. It doesn't quiet feel as fast as the PC version of UT3, though--Epic commented prior to release that the speed of the PS3 version was slowed down slightly, and that seems to be the case. It doesn't bother me at all. (Note that I am not indicating there is any kind of framerate "slowdown" or other performance issue; it is the pace of the game itself to which I am referring.)
Still, you can turn the controller sensitivity and acceleration up to a ridiculous degree, moreso than most console games. Of course, if you're at that level, it's probably time to learn to use a mouse.
You can use the Sixaxis' motion sensing to control the hoverboards, but I'm not sure why you'd want to. Even as a novelty, it's unwieldy.
Main vehicle control is customizable, but thankfully defaults to Halo-like relative steering, which works much better with the controller.
Server browsing is similar to the PC, with the ability to filter by various game options. You can specify whether you want to restrict your choices to dedicated servers. When creating a multiplayer game, you can choose to host as a dedicated server, and dedicated PS3 servers can be run from PCs as well.
User mod and map content is of course a feather in UT3 PS3's cap, being a practically unheard of feature on consoles. PC users can create content and export it for PS3, then load it onto a USB drive or PS3-compatible memory card (SD, CompactFlash, or Memory Stick as long as you've got the right PS3 model) and import it in the game. You can also download mods directly from the dedicated servers using them. Since the game isn't technically out yet at the time of writing, I wasn't able to find anything, but it's an appreciated feature and one that should help with the game's longevity.
Frankly, it wouldn't surprise me if UT3 on PS3 outsold the game on PC--at least initially. The strong existing PC community will be a huge asset on PC long-term, but right out of the gate UT3 occupies a unique space on PS3 and has a lot of hype among PS3 gamers looking for titles that make their console stand out.
Meanwhile, PC gamers have a lot on their plate and UT3 hasn't generated the same hype the series has in the past. That said, it's a great bit of old-school fast-paced FPS action that will be fresh to console players and seems like a good alternative option for existing fans of the series whose PCs aren't up to snuff.