A massive new update called "Mann vs. Machine" is scheduled to be released on August 15.
For two years or so, Valve has hinted Mann vs Machine - or MvM - in several updates and hidden links throughout the game as well as the TF2 website. Yesterday, they released a comic that continued the game's wacky background "story", killing off the Mann Brothers, the old behind the RED and BLU teams that keep fighting battles over nothing, and introducing a third brother that Redmund and Blutarch Mann did not know about.
According to web-released comics by Valve, Gray Mann (G. Mann; reference to HL2's G-Man?), the third brother, is launching attacks on the RED and BLU teams' bases everywhere. These attacks involve some scary-looking metal robots that resemble the game's characters. As with the recent Pyromania update, Valve is teasing its players an releasing information about this new update over a period of three days over at http://www.teamfortress.com/mvm/. The MvM video is embedded below.
The Mann vs. Machine update seems to be some sort of co-op mode (in new maps!) that gamers will play with five of their friends in which they will stop the robots from deploying a bomb on their base (hopefully there will be automatic match-making so people without friends can play as well). The comics mention something about the robots running on money, which the players can pick up to upgrade their defenses. This is basically a zombie apocalypse style stronghold defense mode, but with robots. We will learn more about this over the next two days.
Online communities like the /r/tf2 subreddit on reddit.com have been following Valve's hidden goodies in the game updates and their 3D animation program, Source Filmmaker for a while now. The announcement of this update is a huge deal for the entire community. TF2 has a huge fan following.
UPDATE: An MvM playtester is answering questions about the new update.