This is a great idea. The point is merely to provide an alternative to an explicit CAPTCHA that requires interaction like reCAPTCHA that doesn't cost less to try and break. That way, it's strictly more convenient to the user, if they use autofill or whatever, they'll just be subject to the ordinary reCAPTCHA that they would be anyway.

I'm imagining listening to mousemove on the document and doing something lightweight like appending the coordinates to an array onmousemove. Then a function that's setInterval'd every 2 seconds or something would flush the array to the server via XHR. A server-side script would, transparently to the user, analyze the mousetrail and if the user appeared to move their mouse realistically and took at least like a minute to fill out the form and took realistic amounts to type in text inputs and so on, the server will let them skip the reCAPTCHA.