FWIW, I just checked what the RationalWiki IP-edit captcha does: we're using QuestyCaptcha, which asks "What is the nth letter of our name under the logo?" (said name being of course "RationalWiki") - and this (a) utterly confuses the spambots (b) doesn't hamper accessibility.

Edit: Although of late we have a vandalbot written specially for us, which hops from open proxy to open proxy. So an arms race over the questions would be laborious - thinking of good new quesitions is slower than telling your bot how to deal with them - and our work is to find what proxy list he's using and inform the blackhole list operators of it.