I don't have much to add as far as how well optimized the UT3 engine is for multi-core/multi-thread processing, but I can give you some food for thought, for those that are concerned with what processor they should get:
I am running an old Core 2 Duo (E6600), overclocked to 3.4ghz, with 2GB of DDR2800 RAM, a GeForce 8800 GTS 512 (G92) slightly overclocked to 700mhz core and 1000 mhz RAM, and a GeForce 9600 GT 512 as a dedicated PhysX processor... I get a very playable experience (I have not measured FPS yet, but if I had to take a guess, I would guesstimate 30+ at almost all times, and never really dipping below for more than a second or two at a time), with all settings on medium (but FxAA set to low, and resolution set to 1280x720) and hardware (gpu) physics handling enabled.
My system is far from a cutting edge multi-core system... so I would have to imagine that anything from the current generation of multi-core processors would have no problem handling this game beautifully. Remember, it's not so much your processor that is going to be holding you back as much as it is your GPU(s), unless you are letting your CPU handle the physics. In that case, then yeah I guess you want something beefier... but honestly, even my old C2D did fine when I ran it with CPU physics handling.
On a completely separate note, it's also worth pointing out my specs and performance to show that this game can, indeed, be played acceptably on a slightly older rig (as long as you are willing to make some concessions to your settings). Protip: using Ultra settings does not make you any better at the game than someone who uses Medium settings... it also doesn't make the game any more or less enjoyable, unless you are obsessed with everything being completely maxed out for some reason. Yes, it looks beautiful maxed out, but I would much rather sacrifice a little bit of eye candy for a lot of performance.