I wrote code to do that for a column of mine, but beware that I took a lot of flack for that column.
in reply to Code to Block Scripts/Harwesters (GD based?)
In particular, it seems that if a website uses this technique, but doesn't provide an adequate backup plan for blind people, then the site may be in violation of the US ADA rules, which applies to all government sites, government contractors, and anyone with deep enough pockets. Southwest Airlines lost an ADA case for their website recently, for example.
So, while you may not want robots stuffing your forms, you must also clearly provide a means by which visually challenged humans can still take action at your website.