An example of an ARG like game played out in a 1997 movie starring Mike Douglas, Sean Penn, Deborah Kara Unger, et al. Could be watched & studied for various examples of ARG game play & story elements.
Along these lines, I think there are probably lots of movies & books from which inspiration could be drawn for an ARG, e.g. genre, storyline or plot development, setting, etc. For example, it occurs to me that another one of my favorite films, Dark City, could also be great inspiration for an ARG with a bit of sci fi noir flavor to it: http://en.wikipedia.org/wiki/Dark_City_(1998_film)
Borrowing from the concept framework provided by the Wikipedia entry for Immersion, I think an ARG must have at least the characteristics of emotional/narrative immersion and cognitive/strategic immersion; basically connecting to a participant's interest and level of focus/engagement. From my perspective, the emotional/narrative component is key, because I don't think the act of just tackling a series of puzzles makes something an ARG.
And if we then add blended & augmented reality elements, then I think there's the opportunity to allow for sensory-motoric/tactical immersion as well as spatial immersion.
Wow, here's a gameplay clip that captures all three aspects of the immersion experience, sensory-motoric, cognitive & emotional; and it somehow even manages to throw in a bit of cognitive dissonance for extra measure... I think it's a great example of the impact creating these kinds of 'ah ha' moments can have and how these moments can distinguish an ARG as an engaging learning platform: http://www.youtube.com/watch?v=Y8AFiaWz7WI
Thought this content could be helpful from the standpoint of thinking about the psychological aspects of play and how they might apply to design & development of an ARG, as well as the playing of ARGs.
This behind the scenes retrospective look at Perplex City ARG shows some of the elements that went into that game and might offer some interesting ideas....
Thought this content could be helpful from the standpoint of thinking about the different types of immersion and their elements; and how these might be used in the design of an ARG.
The ARG SIG Wiki of the International Game Developers Association looks like it could be a rich source to mine for info, especially their white papers...