Lol, dopey. Here's a quick update that may or may not help you. I recently reinstalled Mail (if you check out the threads, you'll find my Mail "problem"), and after I had to create a "new" (the 'welcome to mail' prompt appeared, and I had to recreate my old account) account for myself, I figured that I'd have to reimport all my old stuff (by old stuff, I mean all the mail I've received since I was careless enough to not back up my previous emails T_T). So I did, and aside from having some problems with mail boxes that have the same names (inbox, outbox, etc.), it worked. I was able to import my mail messages from the '/users/username/library/mail' back into mail, under a new mailbox. I was just about to drag the respective contents into their respective 'boxes, when I thought to check the new inbox and sent items, and lo and behold...my stuff was already there (they must have been reloaded as soon as I finished re-making my new account)!
Anyway, beg pardon for the long-winded story. Bottom line: I was able to import the contents of the '/users/username/library/mail' back into Mail. I only did it once, but if it worked for me, there shouldn't be any reason that it won't work for you.
Hope this helps.
peayce.