I thought about this idea back for a certain pfw as a mechanism to allow flexibility in the display of the form tags. For example, allowing one to have a renderer that would output a calendar interface instead of a date entry if configured to do so. The renderers could be just for the input tags really, and would be the equivalent of the UI classes in swing.
What struck the idea up again was this other wf: http://examples.millstone.org/features/ (check out the DateField component, and set to calendar rendering)
I also really like the idea of being able to intercept the rendering to show, for example, required field icons/stars, perhaps by groking the destination validation info. Or just a JS info button that explains the field, based on a property lookup.