E3 team, since you're doing work on the registration/login screen - this might interest you :).
Note the big gotcha he describes, which very much applies to us...
"It is still possible for someone to write a bot to exploit a single site by closely examining the DOM. This means that if you are Yahoo, Google or Facebook, negative captchas will not be a complete solution. But if you have a small application, negative captchas will likely be a very, very good solution for you. There are no easy work-arounds to this quite yet. Let me know if you have one."
In short, I think this would probably increase the effectiveness of our efforts to combat bot registrations, but would likely not be a replacement for our current CAPTCHAs. Chris Steipp would have more to say on this front, I'm sure.