Just a little quickie (but the little ones are usually the nicest!)...double click on the square just above the scrollbars but to the right of the message header titles (e.g. Date & Time on my std install), and Mail toggles from Preview Message to List view (or I guess more accurately, toggles preview off and on). Same happens if you double click on the separator bar between the lists and the preview.
Still a little light on functionality, and a bit slow when you have more than one mailbox (try 6+), but hey it's only a Beta oops i mean 1.0 release.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2001052402403827