gellyfish explained some of the problems with using CAPTCHAs last month.

Specifically, one of the links he sends you to discusses attacks against CAPTCHA by tricking humans into doing the work -- and if the goal is to let humans in, you're always going to be at least somewhat vulnerable to it. (we assume that they're using onion routing or something similar to keep you from limiting their submissions by IP).

Captchas just make things more difficult to abuse, not impossible to abuse. There is no perfect solution, and you risk leaving out legitimate users.