Paul Trevethick has put together a document identifying the dimensions along which components and data flows can be changed in user-centric identity systems. His space is a diagram that is general enough to cover most wire-level interactions of various user-centric identity systems. I found it instructive. For any specific set of interactions with various components some of the components or flows would drop out.
This is timely because we're trying to figure out how to do interoperability demonstrations for IIW07 in May. That requires mapping out scenarios that various parties will try to play in. Paul's diagram gives a good indication of the possible space of interactions from which those scenarios can be drawn. The hope is that this interaction will lead to an understanding of what's possible and what's not working yet.