WCF REST Service not accepting JSON in .Net 4

I've been trying out the StandardEndpoints that were introduced as part of .Net 4 and I'm getting the most peculiar of errors.

My code

[ServiceContract]
public interface IAuthenticator
{
    [OperationContract]
    [WebInvoke(UriTemplate = "AuthenticateUser", Method = "POST", BodyStyle = WebMessageBodyStyle.WrappedRequest, ResponseFormat = WebMessageFormat.Json)]
    AuthPacket AuthenticateUser(string Username, string Password, string DeviceId);
}

My web.config

<system.web>
  <compilation debug="true" targetFramework="4.0" />
</system.web>

<system.webServer>
  <modules runAllManagedModulesForAllRequests="true">
  </modules>
</system.webServer>

<system.serviceModel>
  <serviceHostingEnvironment aspNetCompatibilityEnabled="true"/>
  <standardEndpoints>
    <webHttpEndpoint>
      <!--
          Configure the WCF REST service base address via the global.asax.cs file and the default endpoint
          via the attributes on the <standardEndpoint> element below
      -->
      <standardEndpoint name="" helpEnabled="true" automaticFormatSelectionEnabled="true"/>
    </webHttpEndpoint>
  </standardEndpoints>
</system.serviceModel>

The exception that is driving me crazy!

415 Cannot process the message because the content type 'application/json' was not 
the expected type 'text/xml; charset=utf-8'.

I can make the problem going away by going back to the .Net 3.5 standard of declaring each service, but, unless I am mistaken, one of the major upgrades in WCF with .Net 4 was it's ability to handle stuff like this. Am I doing something wrong?