The problem is that problems that are easy to generate with a computer are usually easy to solve with one. Even if you rotated through 100 different variations, solving 10 of those would give the spammer a 10% success rate. If it is just for one small site, no one is going to break it, but there are easier ways to prevent spam on small sites (comment spam bots don't understand JavaScript for instance - something that I take advantage of on my blog).

A system like that might be good for keeping commenter quality high though :).