How to use System.Json in non-Silverlight .NET apps ( .NET 4.0/ WPF)
Got a nice implementation https://github.com/JeffreyZhao/System.Json , which is 100% compatible with System.Json.dll worked like a charm! I didn't need any code change in my Silverlight VM to port to WPF.
Although this will most likely mean some changes to your deserialization code but the Json.NET framework is available on WP7, Silverlight and WPF.