I posted earlier that we're working on a client-side MVC framwork that compliments the ASP.NET MVC framework. It's actually not formally MVC on the client, but it comes close. The view side is entirely controller-driven, even the event model. The "model" side of it is the ASP.NET server itself. So the server spits out both templated markup for the client-side controls, as well as the AJAX'd data models. These data models are models to the client but they are views to the server, or at least that's the way we're designing it.
So my co-worker posted his solution to XML and JSON callbacks, since ASMX is not MVC-friendly. It's a JSON view engine for MVC.
"I recently saw Scott Guthrie at a local .NET User Group event, and he mentioned that the ASP.NET MVC Framework was completely "pluggable", and you could easily replace the built-in View Engine. So, I immediately searched for examples, found one ... tossed it out and whipped together a really quick and simple 'JsonViewEngine' class. Here's the code ..."