I'm helping a colleague of mine get into a RIA start-up and the original development he used ASP MVC on Windows. Do you know if any of the RIA JS frameworks play nicely with ASP MVC?
It feels like a nice fit to MVC/MS stuff (there's certainly plenty of examples of this, and Steve Sanderson now works for MS). And it's easy to bind whatever UI you want to your view models.
Personally I now use KO for even the smallest of JS/HTML 'apps'. It's lovely :)
ControllerName.FunctionName(arg0, arg1, callbackFn);