The real solution might be to have Captcha technology built right into the browser that can physically detect whether or not a human is interfacing with the form.
There are just as many holes. How does a browser detect whether a human is using it? Even if it could how would the site be made aware of this? Does the browser send some special information with the request? If so, can't it be reproduced by some script?
The browser can be scripted to mimic human behavior -- clicks, keyboard entry, mouse movements, etc. How do you determine that a human is behind the browser?
