We only have one quad core machine in the office, and it plays the game very quickly indeed.
We haven't actually tested "core usage" at all to see how many it's using, and it hasn't been specifically optimised for quad core PC's. Some of the work on the 360 version will use mutliple cores, and that might translate into multi-core use on a PC aswell as some of that code will have been transferred over.
So whilst we've got no specific benchmarks, or specific optimisations done on purpose, multi-core machines do seem to play the game faster. We set up a game with all leagues running very quickly (I won a bet against Kev over the time it would take to set up

).
That said, if you don't notice a difference, don't blame me, as we've done no specific tests - it's just from personal experience.