I've implemented a SpamAssassin plugin for WordPress which speaks the spamd protocol natively without any help from outside programs. It should work on 1.2 and later; I'm using it on 2005-01-02 with no trouble (so far).
I know there's already a hack for this, but it requires Perl and some Perl modules and that's not an option for a lot of people.
This is completely bare-bones so far and there's a lot of room for improvement. Please let me know if you find any unknown bugs with it.