We enabled two new key extensibility points in VS 2010 to better handle rich post bodies that we often see with “web 2.0” style apps and with SilverLight. The first is you can create custom editors for the request body: Request Body Plugins (also here: How to: Create a Custom HTTP Body Editor for the Web Performance Test Editor). The second is you can add new tabs to the Web Test result viewer: How to: Create a Visual Studio Add-In for the Web Performance Test Results Viewer.
Mahipal Kante, the dev lead for load test, leveraged these extensibility points to build a great set of plugins for handling the WCF msbin1 format and JSON format. It enables both editing the parameter values in the editor as well as viewing them in the results viewer.
The beta is available here: http://teamtestplugins.codeplex.com/releases/view/43166.