Your best bet is Teo Lachev's forms authentication examples. Try a search for Teo and/or Proligika. His articles have appeared on DevX.

Of course the built in example from Microsoft is the best starting point.

Be sure to look at the example extensions when installing RS.

Brian Larson also has good examples in VB. Teo is a C# guy.

However, Teo dives into the use of not only users but groups for your own custom security extension.

Note I'm not certain in fall 2008, given the massive changes in IIS/RS if custom security will still be needed.