Isn't this basically the honeypot captcha or invisble captcha method? I don't think it would work as well since we our forms are submitted via ajax request, but I could be mistaken?
It's my understanding that the spammers parse the form's HTML to determine which fields must be submitted and the URL to which they must be POSTed.
Remember, you don't need a perfect solution to this. When you and another person are being chased by a tiger, you don't need to outrun the tiger, just outrun the other guy.
Remember, you don't need a perfect solution to this. When you and another person are being chased by a tiger, you don't need to outrun the tiger, just outrun the other guy.