Valve's Unannounced Top Secret Games

Apparently Valve recently updated the Valve Corporation Website, a far less significant online presence than the Steam website that's home to the public messaging of this very private company (thanks TweakTown). Their "about" page discusses their catalog, past present and future. They list their current games as Counter-Strike: Global Offensive, Dota 2, and Team Fortress 2. Then they talk about their "earlier titles," Half-Life, Left 4 Dead, Portal, Half-Life 2, Left 4 Dead 2, and Portal 2. Finally, and this is where it gets interesting, they refer to new games, saying they have unannounced titles in the works which are "top secret." The two works-in-progress they mention are Artifact and In the Valley of Gods, but as we say, the most intriguing projects are the ones they don't list. Word is:
We have some new games in the works, too. A couple have been announced, while others remain top secret.