Sparked from the discussion on the list, I've put my antispam plugin into git and split up the code into a frontend and possibly multiple backends. See http://git.sipsolutions.net/dovecot-antispam.git/ (URL functions as both gitweb and git URL)
Andreas, I took your README file and adjusted it, I hope you don't mind. I didn't fork from your git tree because you'd already implemented the libdspam backend there. Maybe you could take a look at the new backend/frontend separation and implement a libdspam backend.
Aria, it should be possible to write a spamassassin plugin, quite easily if spamassassin supports training based on signatures, not quite so easily otherwise though I intend to plan this.
Timothy, you could write the mysql logger as a backend too.
I take patches :)
Oh and as always thanks to Timo for implementing such a nice server and string, memory and mail handling API.
johannes