For a project I needed to validate incoming WCF messages against an XSD. After a few minutes I discovered the following MSDN article about Message Inspectors containing exactly the information I needed: http://msdn.microsoft.com/en-us/library/aa717047.aspx.
It shows a sample implements a basic client and service message validation mechanism that validates incoming messages against a set of configurable XML Schema documents. This sample is part of the Windows Communication Foundation (WCF) and Windows Workflow Foundation (WF) Samples for .NET Framework 4.