With the latest laptop models that supports three-fingered scrolling, if you use three fingers on your trackpad and move either up/down or left/right, you can move to the next (or previous) Mail message in the current listing that you are viewing. This makes for an easy way of moving through mail -- lift a finger to scroll the message content, then put it back down and move your fingers to go to the next message.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20080519124309278