Our little experiment at wiki.ucalgary.ca has been having some rough times. It's gotten so frustrating that I'd had to temporarily disable new account creation in a desparate attempt to thwart the automated spambots (which automatically create a new account for each edit so it's harder to roll them back).
I've just updated the wiki to the latest and greatest Mediawiki 1.6.1, and one of the new extensions that work with this version is one called ConfirmEdit. It can be configured to challenge "users" with a captcha upon account creation, as well as on each page edit (even only on page edits containing a URL).
For now, I've set the wiki up so that all edits require an account (no anonymous edits - I can live with that), and all account creations must provide a correct simple captcha response. Once you've created an account, you should never see the captcha again.
I'm settling for the "SimpleCaptcha" implementation - it's a math question rather than a nasty wordimage. As much as I hate captchas is general, I figure it's easy enough to fire up Calculator.app when you create an account, and shouldn't block anyone with visual challenges.