I’ve seen ideas similar to that before. I think [PlayThrus](http://areyouahuman.com/ “PlayThrus by Are You a Human”) are a slightly more elegant approach (despite the obnoxious name).
I’ve also seen a lot of people merely an input that says something like “Which is warmer, ice or steam?” From purely a useability perspective that isn’t the worst thing you can do, but it won’t necessarily stop all the bots either. Then again, neither to Captchas.
What we really need is for some brilliant person to figure out how to stop the bots without needing to ask your users for anything. Maybe something like Jeff Starr’s [Blackhole](http://perishablepress.com/blackhole-bad-bots/)?