Thinking a bit more about wiki spam avoidance: from a usability point of view, it’s a bit annoying for a user to have to enter a code they see in an image every time they edit a wikipage. The system does pretty much completely block out bots though. That’s a good start.
So why don’t we set a cookie after someone has entered it once (and thus proven they’re not a bot)? After that they won’t be challenged again. Cons: a real person could still run a script after going through “the barrier” once manually. Maybe the cookie could be turned off every time the person enters a link to another site, thereby identifying themselves as a possible spammer. I assume maybe 80% of all wiki page edits don’t add links to other sites.
Whatever the solution, avoiding spam is going to be important for any wiki. The potential for abuse is just too great. Until I come up with a solution (I am seriously thinking about working something out), I just monitor my recent changes by RSS and keep an eye out for wikispam.