I wrote up an article on fighting forum spam earlier today: http://www.securitywatch.co.uk/forum-spam/how-fight-forum-spamming-222.html However, I figure there are other suggestions and recommendations I can incorporate into it - so if anyone else has anything useful to add please do feel free to post it here, and I'll keep the thread updated. Also, if you feel the article is useful, you're more than welcome to Shout or Digg it.
you covered some stuff I wasn't really aware of (the homepage thing but never posting, for example), but you missed I think the most significant one, at least for every forum I've run: ditch captcha. go with a custom question validation instead. hack for it here: http://www.vbulletin.org/forum/showthread.php?t=124828