After my post announcing my Provider Model presentation in Kansas City, ScottGu commented about the ASP.NET Developer Center's Provider Toolkit available on MSDN. This is an excellent resource to get more information about Providers and examples for ASP.NET. If you are dabbling in ASP.NET 2.0, this is definitely a page to bookmark.
Two great articles on Extending Providers:
Understanding and Extending the Site Navigation System in ASP.NET 2.0
Custom Site Map Providers in ASP.NET 2.0