I have mouse lag myself, sometimes more than other times, it seems to be very related to what my fps is at.
Running vista x64, and the game runs okay.
I know the coolbit tweak and turning vsync of has helped me before, but in vista the coolbit tweak dosen't work, and also there is been recently known that forcing vsync off trough the Nvidia control panel doesn't work because of a limitation within vista or something, only way to turn vsync off is by using ingame settings, and an ingame setting for vsync in ArmA is not existing atm.
vista x64
amd64 3700+
nvidia 7800gtx 256mb
2gb ram
creative x-fi xtrememusic