Vittorio Bertocci has a nice high-level walkthrough of performing federation in a web service using the Geneva framework.  Federation is a popular way of solving the trust problem when there are many different authorities of trust that an application has to deal with.

• ASP.NET, Geneva Framework & FedUtil.exe
• Using Geneva Server for adding a Relying Party relationship to the Identity Provider

For those interested in a more technical look, Dominick Baier has a separate pair of articles on how Geneva is integrated into WCF and ASP.NET.

• Geneva integration into WCF
• Geneva integration into ASP.NET