The gist of Fitts's Law is that the time it takes to point to a target - in our case here, with your mouse or trackpad - is a function of the distance and size of the target. Bigger and closer targets are easier to hit.The beauty here is in how Apple has leveraged this concept with Mac OS X's UI, right down to the reason behind the menubar being pinned and always accessible at the top of your display. Basically speaking, there are five primary targets that are dead simple to hit, without even having to look at them: the four corners of your display and the menubar (and sure, the Dock could count as a sixth, but I'm leaving it out for now). You can easily just fling your mouse 'up' and hit the menu that governs the application you're working in (or any you can quickly switch to); it might seem like a minor detail, but it's one that makes it a lot easier to land on the menu you need and keep working.
The four corners of your display are even easier, according to Fitts' Law, since they are what John called 'infinitely large targets.' You can close your eyes and fling your mouse 'down and to the left' and know that you've hit the lower left corner of any display. Attach an action like invoking Dashboard or an Exposé view to those corners (accessible via the Dashboard and Exposé System Preferences pane) and you have just enabled a powerful way to access information and organize your windows. Tack on a 3rd party tool like MaxMenus, CornerClick (a download is available but their site is under renovation) or Spanner and you open up even more possibilities for using these incredibly easy targets to enhance the way you work.