I tested it a few times in their demo trying hard to to funny things with the mouse pointer. Still I was asked to type in some text all the time.

Then I logged into my gmail account and yes it worked.

So you're probably right about that smoke screen and it has nothing to do at all with mouse movement.

My fx browser deletes cookies at exit and my IP changes frequently and I think that's the true explanation for the outcome of my little test.

I was confused about all of this until I read your comment. Makes perfect sense now!

Also makes me think that they can dump the whole charade and provide no security check at all, but that'd probably make the service-providers uncomfortable and they lose the user as a source of human-intelligence for classifying things on google image searches.

I'm not sure where cursor movement was mentioned, other than the comments.

I read it in the wired.com article about this also submitted to HN: "And [Vinay] Shet says even the tiny movements a user’s mouse makes as it hovers and approaches a checkbox can help reveal an automated bot."

Sounds like if I 'Tab' to the field & hit enter I must be a bot....

I guess it's a lot more complicated than it looks at a cursory glance.

It's almost as if Google is playing its cards close to its chest to avoid tipping off people intent on defeating it's captchas!

Which is why Google Analytics is free for site operators.

To be fair, any service or site - Facebook, Twitter, Google Analytics, etc - that encourages your putting their code or widgets or whatever on your site is doing exactly that in addition to listing out which of your friends "like this".

Makes perfect sense. This is Google's response to the Facebook 'Like' button [tracker]

Then what is Google Analytics?

can be blocked without any impact on the browsing experience.