ASP.NET MVC2 and JSON model binding with validation to an action method
For MVC2 you need Futures. Get the dll, add reference to it and in Global.asax add (Application_Start):
ValueProviderFactories.Factories.Add(new JsonValueProviderFactory());
Don't know about MVC3 - I'm waiting for an RTM.But I do encourage you to give it a go, as sending JSON up to Actions is a pure bliss ;-)
In MVC 3 there is ValueProviderFactories provided out of the box.