I've mentioned your work every time I have come across the discussion of CAPCHAs, and have been quite surprised at the lack of enthusiasm others have greeted it with. I've implemented the idea in (highly-non-portable) PHP twice to serve my own needs, and was impressed both times with the effectiveness of the method. This is an elegant approach that just hasn't tipped yet.
Actually. there is some irony here: the recent upsurge in spam seems to be utilizing this very idea to defeat filters. Anecdotally, I've had a number of junk messages show up in my Gmail account lately, which usually remains pristine. It looks like somebody is using the idea: the spammers!