Seems like yet another application of Luis Von Ahn's notion of "human computation". Using human cycles to solve computationally intractable problems:

http://tinyurl.com/2wtrco

Its interesting to note that given the usability constraints of captchas people will almost always go for a "more usable" and algorithmically complex alternative if one is available. The use of akismet to stop comment spam in blogs in a case in point.