Just an idea for a new captcha system. How's about if you could access a large photographic resource (e.g., Flickr, Picasa), and a thumbnail of the picture is shown to the user. The user then has to guess one of the tags belonging to the picture. If they do, then they pass.
This cannot fight against brute force (I would imagine some tags will be quite common so some research would produce a frequency-based word list that could do this), mistakes would be expected (eg, the photo owner might put in random tags that make no sense to anyone else) and human-based captcha solving will easily get around it, but it's something to consider. Another crack would be to take the thumbnail and compare it against a DB of flickr pictures, but realistically, that's a large job. I wonder if Flickrs API can do that? To defend against this, the thumbnail could be altered somehow (eg, desaturated, change the colour balance etc) so that in machine terms, the images are different but in human terms they mean the same thing.