I like this sort of approach for community oriented captcha. Ask a question that anyone within the community would know or could easily find out but a general purpose spam bot or average person would be unable to solve.

If the community is profitable enough or becomes big enough some spammer will spend 15 minutes or 50 dollars on mechanical turk and find all the answers. As others have mentioned it is an arms race in the end. Back and forth each side upping the challenge.