A friend of mine had a go at building such a puzzle to deter automated downloads of activation codes. He built a prototype. It was just a pattern matching problem. It would take a set of numbers draw them in a pixel array and munge it around a bit, scatter random pixels over it.
Humans can easily read off the number.
It would take a bit of effort to build OCR software that can read through that noise. But I'm pretty sure it could be done if somone were motivated. You see a similar thing where people render their email addresses as a GIF to hide it from spiders. It seems not so hard to me.