42 Entertainment is a pretty fascinating company.
They're best known for the "I Love Bees" ARG which launched Halo 2, as well as the "Why So Serious?" work they did as a teaser for The Dark Knight.
They're also behind the "Flynn Lives" site that's teasing the new Tron movie.
If you go on their home page, you can opt in to follow them down the rabbit hole into their own ARG by providing your email address and phone number to receive text messages and other hints.
While their work is frequently featured in Creativity and other pubs of the ad industry, I'm not sure you'd call them an advertising agency. Their Wikipedia entry says they specialize in "creating and producing alternate reality games." Which means they know how to work online and do ambient/guerilla media - two areas traditional agencies have been trying to master themselves.
They probably don't pitch for accounts against traditional agencies like Chiat or Goodby. In fact, I wouldn't be surprised if some clients would work with 42 Entertainment and their agency of record on certain projects.
So while they may not be the next evolution of ad agencies, they're definitely a social media-catalyzed mutation. And their Grand Prix win at Cannes this year shows that mutation is pretty legitimate.