With more and more tools available to monitor mouse movement, like RobotReplay, the question of whether mouse tracking can be substituted, or at least partially replicate, eye tracking is active.
This eye tracking video of Squidoo by ETRE demonstrates both times of synch between eye and mouse and times where they diverge. If you watch carefully, you'll see that given an intent / opportunity to click in the current user activity, the mouse is much more likely to be close to the eye.
This is a basic rationality that says "If I might click, I might as well keep the mouse close to my eyes." Where there's no potential to click, either because the user is in an evaluative mode or the content of interest is devoid of links, the mouse and eye diverge.
The scientific literature is closing in on this interpretation with results which show that mouse position is predictive of gaze, sometimes. How this partial mapping can best be turned into value is to be determined, but take a peek at robot replay's demos as watching the user's mouse interact with the page does create a powerful interpretation of the experience.
Update: Summary of research on eye - mouse synch available from: Edmonds, A., White, R., Morris, D., Drucker, S.
Instrumenting the Dynamic Web. Journal of Web
Engineering (JWE), Vol. 6, No. 3 (2007), 243-260.