The man who brought us the original ARG's speaks up.
"I consider the first ARG The Beatles’ “Sergeant Pepper” album. An alternate reality game is anything that takes your life and converts it into an entertainment space. If you look at a typical video game, it’s really about turning you into a hero; a super hero, a secret agent. It’s your ability to step outside your life and be someone else. An ARG takes those same sensibilities and applies them to your actual life.
...They embedded a bizarre series of clues and mysteries into the cover, turning you into something more than you previously thought of you were. Look at this album: There’s a phone number hidden in the bushes over there. What would happen if I called it? Your superpower is simply that you notice this cool thing that most people don’t notice."