This approach, using browser as a general GUI framework and shipping with an embedded server, has certainly been used before. I'm not sure whether there have been any established general framework, or whether this is really different from any web application.
There are certainly some limitations for this type of application, it needs to be web-like (e.g., pretty difficult to write a drawing app). The recent talk about AJAX (as exemplifed by gmail and google maps) certainly make this approach more like a desktop application.
Futhermore, the GUI framework in Firefox is another nice approach (XUL and javascript), not just for firefox extensions but as a general GUI framework.
