Actually, this is exactly what the OpenThought Application Environment was designed to do. Using an event like JavaScript's onChange(), you can send parameters to the server. The server can then respond in some way, and update portions of the existing document, all without reloading the page.

There is a demo on the OpenThought website which will show you some of it's capabilities. One thing the online demo doesn't reflect at the moment is that OpenThought isn't limited to updating form fields, it can also dynamicaly update portions of HTML.

The code for that demo comes with the OpenThought distribution, so you can use that to help you get started.

Good luck!
-Eric

--
Lucy: "What happens if you practice the piano for 20 years and then end up not being rich and famous?"
Schroeder: "The joy is in the playing."