I saw a website that actually did have a captcha like this, but I do not remember which off-hand.

However, just like ever other proposed captcha replacement it has many issues. Someone should make a 'Your new captcha will not work because...' checklist like the one for email spam.

edit: http://theymakeapps.com/users/add is the example of a slider captcha I was remembering. It was previously discussed http://news.ycombinator.com/item?id=1479411.