Oddly enough, stopping and restarting Synergy fixed it.
(I read it on another forum) So I'm back to normal.
Thanks,
If this is at all helpful, my system was working fine (Windows 7, USB mouse) for months. Then I unplugged my mouse while working on something. When I plugged it back in and went back into Minecraft this problem started. Same computer, same mouse.
I noticed if I don't move (walk forward or back) the mouse works fine, as soon as I take one step in minecraft it goes into super sensitive spinning mode.
Still happens in 1.6.2.
http://synergy-foss.org/