There was once a hint that worked for previous versions of Mail about three-fingered scrolling to flip through messages.
With Lion, the three-fingered flip now launches Mission Control, so navigating through Mail messages has changed to a FOUR-fingered flip. It took me a while to figure it out on the Magic Trackpad -- it seems to call for a little practice.
A flip Up/Left brings you to the next message up the list; a flip Down/Right brings you to the next message down.
[crarko adds: This is starting to take on the feel of learning to play a new musical instrument, which is not all bad, and possibly intentional on Apple's part.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20110729110331981