My bbpress forum is now averaging one registration spam per day - where a user registers, adds their pills or porn site details to the website field, then never visits again.
Check your registrations guys 'n' gals, you may find you have a lot of unwanted members (*cough*) linked from your sites.
I'm thinking it's time registration requests went through Akismet - but for this to happen there would have to be a UI for spotting false positives.
See also: Links to bozos and blocked users.