Solution? Elegant, simple, and (as near as I can tell), undocumented. Simply take a message in the inbox, and drag it towards the left edge of the screen. The drawer will magically switch sides! Mail.app will remember this setting the next time you launch it, too.
Thanks for this tip go to 'cricket,' who works for Apple on the mail team and hangs out on the 'X4U' mailing list, hosted by The Macintosh Guy (also in Portland, I might add - it's a hotbed of Mac addicts!). See the links area for a direct link to the mailing list subscription page.

