SecondLife is a-buzz, but do you know what ARGs are?

SecondLife is seriously hot at the moment. Marketers are really talking about it - only today I was at a major UK brand and the lady I met was really interested in it.
Like all technology adoption, it's working it's way thru the curve, and however passe it may seem to out-and-out web heads, the vast majority (e.g. normal people) haven't even HEARD of SecondLife.

So what about ARGs - do you know what they are and what promises they make?
Alternate Reality Games, apparently.
Not quite a virtual world like SecondLife, or a MMORPG [definition] like World of Warcraft.

I came across the concept on the BBC, reading about the winner of a cutting-edge ARG called Perplex City.

Reading the maker's website got my mind fired.
Fascinating. In some ways more than Virtual Worlds given that they are totally removed, whereas this ARG model blurs the lines between real and virtual.
Sounds like a quick route to insanity, but fascinating fascinating.
It's the future baby.