SSO On WSS R and D

Here are some samples about create customized MembershipProvider and RoleProvider  Using login controls for an existing data source by creating a custom membership provider in ASP.NET 2.0 

http://support.microsoft.com/kb/910440  Custom MembershipProvider and RoleProvider Implementations that use Web Services http://www.codeproject.com/aspnet/WSSecurityProvider.asp  

Also, in MOSS 2007, we can create a Pluggable SSO Provider  completely: http://msdn2.microsoft.com/en-us/library/ms566925.aspx 

http://msdn.microsoft.com/en-us/library/ms566925(v=office.12).aspx

SSO needs a provider(stores & retrieve the credentials) and a consumer(request for credentials). ISsoProvider is the interface for a custom SSO provider. It is used when developing alterative provider to the default one.
 
Please refer: http://msdn.microsoft.com/en-us/library/microsoft.sharepoint.portal.singlesignon.issoprovider.aspx
http://community.bamboosolutions.com/blogs/mashpoint/archive/2008/07/31/article-2.aspx

http://www.networkworld.com/community/node/18791

http://www.endusersharepoint.com/2008/08/31/endusersharepointcom-are-you-using-single-sign-on-with-sharepoint/

http://www.theserverside.net/tt/articles/showarticle.tss?id=ImplementingSSO

http://www.delicious.com/baldwinjoe/SSO

http://www.thorprojects.com/blog/archive/2008/08/02/moss-single-sign-on-setup-step-by-step.aspx