RoboKitten

This would not be hard to do. There are standard names for the auto complete form inputs. The browser (or plugin) could be configured to notify you if some/any of those fields are about to be posted and warn the user using a native code generated pop-up (not a web pop-up).

Modernizr is a great way to be able to start using some of these features with legacy browsers. I actually have to battle with some of my customers to upgrade to IE8 (and not to set the group policy that forces "compatibility mode").