What do you think about services like
Akismet?

What I see is that spammers are always getting smarter, more aggressive, and more numerous. Captcha will not block everything because when spam stops posting, they visit your site to find out why, and then post a few test messages -- including questions to bait you into responding directly by email. Akismet and Bad Behavior cannot block these people either.

On public forums it takes a combined approach: Captcha, Bad Behavior / Akismet, ip blocking, flood prevention, keyword blocking, and member moderators.

That's a lot of work just to stop vandalism. For scripts like the comment script, guestbook, and weblog, I believe the answer is easy: all messages should be subject to approval. On the Forum, the closest equivalent would be to make the forum members-only. I wouldn't be surprised if most forums go in that direction.

Phil