I need to provide a service to a third-party that will be sending soap messages with a signed Timestamp.
How can I configure my service to support this?
You may want to consider a custom security binding class that implements the security just the way you want it, rather than the WCF default.
These MSDN links explain Custom Bindings and the SecurityBindingElement abstract base class:
http://msdn.microsoft.com/en-us/library/ms730305.aspx
http://msdn.microsoft.com/en-us/library/system.servicemodel.channels.securitybindingelement.aspx