In iCal 5.0, which is included with Lion, you can navigate between dates, weeks, months, or years using a finger swipe left or right using an Apple Magic Mouse or Trackpad.
Click on the Day, Week, Month, or Year button. Navigate between dates, weeks, months, or years using a finger swipe left or right. The left or right swipes will advance or reverse the displayed calendar.
With the Magic Mouse use a single finger swipe, and on a Trackpad use a two-finger swipe (you must check 'Swipe between pages' in their respective Control Panels).
[crarko adds: When I tried this with the Magic Trackpad, I could only get it to work reliably in the Weekly calendar view, so your mileage may vary.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20110905084920485