I was discussing this topic with Emmanuel Bernard by email and Emmanuel suggested that we move this conversation to this mailing-list, so here I am  :).

The idea would be to propose a javascript export of the bean validation rules. In that way, it would become fairly easy to do "true client side validation" (that is: validation that does not require access to the server side until the form is completely valid, or nearly completely valid).
Many web technologies such as JSF, Spring MVC, Wicket, Tapestry... integrate with JSR 303. Providing such export would be of great help for users of those frameworks (or standard in the case of JSF).