In the message-list pane of Mail in Panther, up and down arrows will select the previous and next email for viewing. I thought there must be a way to open or close a thread in threaded email view and there is! Right arrow opens up a thread and left arrow closes it, no matter which message in the thread you are currently viewing! Nice!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031022165525297