A Gabe Newell interview offers a clarification of recent comments that "Portal 2 will probably be Valve's last game with an isolated single-player experience." Here's word:

“I think what we’re trying to talk about is the fact that, not that we’re not doing single player games. Portal 2 was a pretty good example of what we’ve learned over the years in terms of how to create those experiences. It’s more that we think we have to work harder in the future, that entertainment is inherently increased in value by having it be social, by letting you play with your friends, by recognizing that you’re connected with other people.”
[…]
“That’s the thing that we’re trying to say, is that, single player is great but we also have to recognize that you have friends, and we wanna have that connected as well. So, it’s not about giving up on single player at all, it’s like saying, we actually think that there’s a bunch of features and capabilities that we need to add into our single player games to recognize the socially connected gamer.”

Thanks RipTen via VG247.