Making sense of the federation protocol landscape

By way of Archie Reed’s Secure Identity Management Blog (Archie is a Director in Identity Management at HP), I just came across this article on Federation by Jason Rouault (also at HP) - Making sense of the federation protocol landscape.
While, obviously, I would direct you away from the section on HP products at the end (), the remainder is a nice summary of federation and a useful comparison of the various protocols.