Eric Norlin has a wonderfully concise post in regards to how we got where we are today in terms of federated identity. I know its a bit dated (as far as IdM technology discussions go) - it was seemingly written as a result of confusions that arose as a result of Higgins being released.
Note: under the heading 'SAML', when he refers to 'web access management' tools - he is referring to tools like cleartrust, siteminder, tivoli access manager, etc.
i think its important to appreciate those tools are really what paved the way for what we have today in terms of federation standards and such. Well, that and a little prodding. Nonetheless, the article gives great context to alot of discussions today regarding attention data, user-centric identity, and stuff like that.