I was signing up to post on a video game forum earlier today (god save me), and in addition to the usual alpha numerics with OCR interferance, they also had a small question at the bottom of the page. It was simply "What's 1+1?" with an explaination that this was a genuine question, as it was so far a bot proof device.

I hardly think we're at the point where we need to worry about that level of bot registration, but it might be worth considering.