It's a sizable read, but Gamasutra's feature on the Portal 2 alternate-reality game is worthy of your attention. Written by Rob Jagnow from Lazy 8 Studios (Cogs), it reveals the story of how Valve got a bunch of indie developers together to make -- and formed a community around -- a compelling ARG.
Most interesting to me is page four, which details what went right, as well as the elements that didn't turn out as anticipated. The final phase (unlocking Portal 2 early), in particular, left a bad taste for some people who tend to get angry about these sorts of things.
"If we had been able to do this all over again, there would have been several changes. First, we would have stuck to the original schedule of having the crescendo be no longer than 36 hours. Given the toss-up between media attention and player goodwill, we'll choose the player every time," writes Jagnow.
"Next, we would have continued to add new in-game puzzle content during the crescendo so that the core ARG players could maintain their excitement and have a way to contribute to the launch in the way that they knew best. One other option that we could consider for future projects is to move a crescendo-style event to the start of the ARG to alert the media and engage our core fans from the very beginning."
The POrTAl TwO ARG: The Whole Story [Gamasutra]