With the U.S. release of Windows Phone 7 around the corner, Caleb Baker of Access Control Services at Microsoft shared a sample that shows how a WinP7 Silverlight application can make use of ACS to call a REST service that can be used millions of users, including those at Live ID, Facebook, Google, Yahoo and AD FS accounts.
In the sample Contoso is hosting a REST service which returns a list of contact information. The phone app is then used to access this service. Users are able to authenticate to the service by signing into one of the identity providers Contoso has configured at their ACS Service Namespace.
Here's Vittorio's PDC talk: