WebServiceHost is a new feature in Orcas that makes it easy to put up simple web services that are built on HTTP and POX. However, there's no requirement that forces you to build REST and POX services using WebServiceHost. WebServiceHost exists to make a simple case easy, but you're not locked into using that approach if the simple case doesn't apply to you.
Here's everything behind WebServiceHost if you want to build your own.
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Whenever my service receives a message the service operation fails because the user identity is not mapped