Take a look at Mudra Mouse. After using it for a while it can become a decent mouse replacement. Of course, it's far from perfect (since Windows is still based around the precision of a mouse, as noted by @jhung), but after tweaking the settings a bit I actually got it to be surprisingly accurate.
There is a trial version for it, though I believe that it will only work in the trial application's window, for OS control the full version is needed.