I noticed it really bad after the 10.6.6 update. So I wiped my drive and started over and have been on 10.6.4. The mouse lag is still there, although not as bad as it was. I dont remember it being there when I first bought my MacPro and was on 10.6.4, so Im shocked that its still there after I wiped my drive to factory settings. I've also done hardware test, reset PRAM and SMC. I've read that it can be caused when iTunes or QT is open but I notice it even when those are closed. Can anyone shed some light on this matter? I'm very desperate.