An article recently reddited on how (seemingly unbreakable) CAPTCHAs such as those used by Google had recently been cracked; As some of the quite interesting comments point out correctly, the so-called "crack" most probably is nothing but an attack by some click-farm. (Check out some of the other hillarious comments, esp. those by click-farmers soliciting their services.)
To me, the most important point is that - with growing popularity - a service inevitably will be exploited for spamming or some other iniquitous use. Hence Captchas can not be more than a significant "speed-bump" to stop full-blown atttacks. They must be combined with other techniques to detect, repel and deter spam and other unwanted usage. This will also be the same for any technology succeeding today's Captchas.
Two further interesting issues:
- Captchas will have to become increasingly hard to break - with a mounting danger of becoming unsolvable. Techniques to break them simply will keep on getting better. For the sake of argument, we might consider Captchas to be a soon-to-be-broken technology.
- Even the Captchas that are in use today are quite a nuisance. And they probably will not be getting fewer in our daily experience.