Blind Crowdsourcing of Captchas

I feel like someone must have thought of this before, but it seems like one way to break captchas would be to create a service that many people want, but needs to be protected against scripted use. Give your users captchas from other sites that you want to break, and they will gladly solve them. You then simply forward the answers along to a script that is doing something nefarious- whatever is it captchas are protecting against. Distributed organic computing for solving the captcha problem.