How do I authenticate to a WCF service via ACS integration with Windows Live ID?

Question

I have a WCF service that uses UserName authentication via ACS. This works great when I'm using Service Identities but when I try to use my Windows Live ID credentials I get the following error:

System.ServiceModel.FaultException: ACS10002: An error occurred while processing the SOAP body. ACS50012: Authentication failed. ACS50026: Principal with name 'louis@arsunica.com' is not a known principal.

Unfortunately I've yet to find an example of how one uses Windows Live ID with a WCF service. The only examples I could find seem to be focused on integrating multiple identity providers with ASP.NET or MVC websites.

Any help in this regard would be greatly appreciated....