I know a few schools out there are working on custom credential providers for Vista.  I have heard of projects such as authenticating to third party kerb realms, etc.  The code samples for Vista RTM have been posted and may be of value. 

Link to Download details: Credential Provider Samples

[Updated 1/3/7 11:45 PM]

I just saw that MSDN magazine has published an article on this same topic.

http://msdn.microsoft.com/msdnmag/issues/07/01/CredentialProviders/default.aspx