I wonder what you think so far.
In particular, I liked the tuple-calculus they define; it can be extended to support just about any permission I can think of (although it does require reversing DNS names, which is slightly ugly).
I have a scheme to release a v2.0 of the standard someday in my copious free time.
All of the resources I've searched for are fairly old, do you have anything more recent that I can read up on? I see a 2006 paper, but not much other than that.
[1] Though I'm well aware that having an open-standard available for a long time doesn't mean squat, as evidenced by Heartbleed-esque bugs.
Edit: Reading the '00 "A Formal Semantics for SPKI" Howell, Katz, Dartmouth. This is what I was looking for.