The Magic Mouse lets you navigate forward and backward between web pages in Safari or Firefox with a two-finger swipe.
The same two-finger swipe also moves you forward and back in iCal. It works just the same as clicking the forward and back arrows around the day/week/month view control at the top of the iCal window.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20091112041516296