I just installed qpsmtpd tonight, it's quite nice.
One piece of functionality that appears to be missing is a useful SMTP auth (besides auth_vpopmail_sql, which isn't useful to me inparticular) plugin. Here's one I threw together that uses my local IMAP server for authentication. If anyone is interested in using it, please feel free.
I figure that since my IMAP server already knows all the authentication mechanisms I'm planning on using, I might as well just let it do what it already does well.
I've only tested this on my own server, it requires Net::IMAP::Simple, and I make no guarantees. I have code for a "auth_imap server foo.com" config option but frankly haven't tested it as it defaults to localhost. YMMV, but it works great for me.