WCF - Third party application authentication

Question

Hello, I am currently working on an iPhone application. This application calls back to WCF services exposed through my ASP.NET web application. Currently, my WCF operation looks like the following:

[OperationContract]
[WebInvoke(Method = "POST")]
public string SubmitMessage(string message, int priority)
{
  try
  {
    // Process message
    // Return success code | message
  }
  catch (Exception)
  {
    // Return error code | message
  }
}

My web application is using ASP.NET Forms Authentication. My challenge is, I only want authenticated users to be able to call this operation from their iPhone. I know that the iPhone SDK has baked in support for XML. However, I’m not sure how to lock down my WCF operation such that only authenticated users can access it.

How do I make my WCF operation only accessible to authenticated users from third party applications?

Thank you