There's now a quick way to catch up on the ARG scene: the first white paper from the IGDA's ARG SIG "provides a full introduction to the genre as well as a wealth of practical and analytical information on design methodologies, business models, and current and recent games."
Though, without too much trumpet blowing, I reckon you're better off starting with an Online (as it then was) cover story, Puppet masters (Thursday December 11, 2003) written by freelance Andrew Losowsky.
It also helps if you own a black Buzz Rickson's MA-1 Flying Jacket -- or at least get the reference.
IGDA = Independent Game Developers Association ARG = Alternate Reality Game SIG == Special Interest Group
