If you organize your messages by conversation (View » Organize by Thread), you can press Option-Space to go to the previous (older) message in the currently-selected conversation (while Space goes to the next (newer) message).
[robg adds: This is different than using Shift-Space (and Space) to navigate the message list -- Option-Space will only work within the current conversation, while Shift-Space goes through the entire message list.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20091014044524861