I experimented by comparing Win 7 and XP x64 last night, as I noticed my friends machine running WinXP was actually running smoother than mine despite his rig being much lower spec.
If you don't want to read all below then to summarise, XP runs this game MUCH smoother.
I was running a few week old Win7 x64 build 7127 and fresh XP x64 SP2. After comparing I then made a completely fresh install of Win7 x64 build 7229 to make sure it wasn't the clean install that was making the difference rather than the OS.
My specs:
Core i7 @ 4.50Ghz
6GB DDR3 @ 1600Mhz
2x ATi Radeon HD 4890 1GB in crossfire
etc etc
Settings: All at highest except view distance 4000, fillrate (render) 100%.
I used the exact same settings and drivers on all installs.
Both the old and fresh Win 7 installations averaged around 35fps and both suffered from stutter.
XP x64 averaged around 50fps and was as smooth as silk. Looks like I'm going to have to keep an XP install just for this game.
EDIT: It seems I'm not the only one that has noticed this. According to
this post on the BIS forums, Win 7 or vista combined with an i7 processor is the problem, specifically hyperthreading.