Captchas are already being broken
Captcha's are at best a temporary solution to a long term problem.
I've followed this topic for a long time.
Do a search and find hundreds of links about breaking captchas.
Some engines are generic enough to work with many captcha generators.
http://www.cs.sfu.ca/~mori/research/gimpy/
http://www.botmaster.net/pictocod/
To respond to this threat, sites make the captchas more and more difficult to recognize to the point where I personally make mistakes when I incorrectly guess what the correct answer is. I imagine an automated program may have a better chance at solving these than I do.
In any case it was long speculated that it would always be possible to pay people to manually solve the captchas.
I would like to see more effort to block spammers at the point of origination.
For instance, apply spam heuristics on a sender's email and take action earlier.
If thousands of accounts are sending the permutations of the same email, then there is a very high chance that it is spam.