I've talked about a metadata exchange approach to inter-federation working here before. Since my last update, I think we've seen some level of acceptance in both the technical and policy communities that this is — at least in principle — a valid approach, and there is work going on in a variety of places on that basis.
One thing that has become apparent as that work has developed is that we need to look at some of our basic assumptions with a fresh eye: complex problems can be often be simplified by looking at them from a different direction. To that end, Chad La Joie (of SWITCH and Shibboleth) and I have put together Interfederation and Metadata Exchange: Concepts and Methods, the current version of which you can download here:
The main aim of Concepts is to provide a framework in which it is possible to think clearly about identity federations in a multi-federation world. This involves first separating concerns and then recombining them in new ways, leading to what we think is probably best thought of as a global metadata layer. There is also coverage of some of the technical implications of such an approach, but we've tried to keep that part as light-weight as possible here.
During the recent Internet2 Member Meeting in Arlington, this document was also reviewed by Scott Cantor, Steven Carmody, Josh Howlett, Leif Johansson, Thomas Lenggenhager and Valter Nordh. We are grateful to our colleagues for their many constructive comments, which we have have tried to incorporate faithfully in the current version. I will leave it to those individuals to state whether, and to what degree, they endorse our conclusions.