EDIT: Nevermind. Still hanging. The pulldowns/etc. are all responsive, it just won't quit. I didn't have this trouble until I setup the IMAP, so I'm sure it's something to do with that...
I just setup several IMAP accounts and had the issue with Mail failing to quit.
It seems to be related to something to do with the automatic checking every X minutes. I set mail sync to "manual" and now Mail quits as soon as I hit quit.
Mike