Here's a quick way to go back and forward while using Safari on OSX Lion (10.7). It also allows you to peek at the webpage that you were on previously.
While using Safari in Lion, use the two finger left or right swipe will take you back or forward, respectively (the direction will be dependent on whether you use natural or reverse scrolling -- I use reverse scrolling). Doing the gesture slowly will give you a peek instead of doing a full back or forward.
[crarko adds: The trackpad on my old MacBook doesn't support these gestures, but if you have a newer machine give this a try, and post results in the comments if you want.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=201107212115506