Anyway, the document itself provides a lot of details about the credprov interface.  Noteably, it includes a section at the end about "Pre-Logon Access Providers," or PLAPs, which is the Vista mechanism for exposing 3rd party, custom network-level authentication prior to (or combined with) the interactive user authentication.  This is the first external documentation I’ve seen about PLAPs.  For example, ever wondered how to implement multi-factor authentication at the network level, while disabling cached credentials?  This is it.