It's ok effective judging by my usages and I could just iterate over this solution if someone brute forced it or implemented a "cat fur detector" (which for my uses is pretty unrealistic).

Battling spammers is really a battle where you implement better protections and they implement better attacks. The conclusion so far has been that it isn't possible to check-mate them - - and it's very unlikely it will ever happen as a lot of spammers use "human bots"...

Using visual captcha is much more user friendly thought, so it's a win for the users.

There is actually a 'Google Tech Talk' on detecting cats (versus other shapes) from images:

http://www.youtube.com/watch?v=-w72_VwSj6A

  Is it really that easy?  I would think flagging for texture would be difficult, as it's
 likely indistinguishable from noise (data, shadows, clothing..).  Approximating shape/avg.
 contour is likely easier than looking for fur patterns.

  Any suggestions on books/papers/etc on image recognition?