Shelley Powers takes an in-depth look at Light-Weight Identity (LID) as well as SXIP. There’s some great stuff here!
[Update: Phil Windley picks up the discussion -- but IMHO he loses it when he starts talking about relationships (Amazon and BYU in his example). I'll assert that most of us would actually prefer what Phil calls 'siloed identities'. I know I would! What's interesting about LID is that I can use pseudonyms easily to accomplish that.]