Photomatt (from WordPress) has built a central anti-spam service called Akismet that can be used with WordPress via a plugin. Additionally, there is an API that allows other services to be integrated. Basically a good idea - even though I generally have an aversion to central services, unless I myself operate these central services.
What really bothers me, however, is this small excerpt from the FAQ:
Well without giving too much of the secret sauce away, we can safely say that it would be pretty difficult to poison Akismet.
So central service - okay. I don't like it, but it certainly makes sense for others who cannot or do not want to operate such a service themselves. But "secret sauce" - I should send my comments with the personal data of my commenters to a foreign system, where I can't even see the software running behind it? Sorry, no thanks.