G2 had an excellent article on Alternate Reality Gaming. It's the genre that the Nokia Game fits in, trails of clues spread across the web and the media. Apparently they started off as fan adjuncts to films etc (the first being AI). But just as viral marketing has commercialised informal viral communications, so ARG has been picked up and used to market movies, games and other "events". There are now even commercial "paid for" games - shades of Michael Douglas' The Game.
A good intro site is www.unfiction.com. They have a brilliant Gamers Handbook that has guides to all sorts of codes and techniques (which very useful on Nokia Game). There's even a Puppetmasters Guide to show you how to set up your own game!
***Imported from old blog***