Since a lot of users in this forum tend to extend the SiteMapProviders or want custom behavior, I thought this might be of interest. We've released the sources for the built in providers on MSDN. Now if you have a question about how one of the providers works or you want to tweak the behavior in some way, you're free to download and hack away. Additionally, you can use the code for reference of good provider designs when building your own.
http://msdn.microsoft.com/asp.net/downloads/providers/default.aspx
One note: The providers availible here are nearly completely functionally equivelent to those in the framework but, due to using internal helpers and other minor technical issues, the sources here aren't 100% identical to the framework. The differences are listed in the details for each provider.
disclaimer: Information provided is 'as is' and conveys no warranties or guarantees.