BBC This is a very crafty way of doing things, and if the spammers get their bots reading words that the current OCR software can't, that also helps.
I heard of this on The Lab with Leo. It is a very cool way to translate books. Captchas don't always work because people take that Captcha put it on their site and get their members to translate it for them then the bot just uses that to get on the site.
You need a reasonably active site to bypass Captchas this way, otherwise you won't get someone entering it before you hit the timeout. But the more people correctly entering the Captcha code (via whatever method) the more help the archivists are getting. I like the idea of tricking spammers into helping me.