For me, this is an interesting and open question. I've taken the view that as long as it's easier to come up with new types of challenges than it is to create an automated means to solve those challenges, CAPTCHAs offer some protection. If you're ok with requiring JavaScript, mouse input, and images (e.g. you don't require your site to be accessible by sight-impaired users who cannot use a mouse), I think people can come up with varied enough challenges to present a problem to bots. How long will that be the case? I don't know.