Valve has teamed up with Adult Swim to create... something based on Team Fortress 2. Little is known about the collaboration, and it appears we may not hear more until "next week," when the promotion goes live.
The promo site simply teases that Adult Swim and Valve "are teaming up for something you'll probably enjoy." It adds, "to learn how their video game peanut butter will be getting in our network's chocolate, come back next week."
The Verge postulates that the partnership could be related to the various character videos released by Valve thus far. "Meet the Pyro" is the next video scheduled for release, and lo and behold, the Pyro appears on Adult Swim's newsletter sign-up page.