Big studios don't take chances and don't make unique game play. That is why the industry always needs people like Notch to take some risks. I wouldn't be surprised if other studios are currently trying to duplicate his game play, but most of the time when this happens, it comes out half assed and worse than the original.
I do think Notch should be working with Valve though. I wouldn't be surprised if he had been offered a full time job there. But he is making his own company and I think that is a huge risk, it could pay off, it could fail miserably. I think Valve would have done for this game what it did for Portal.
Whatever he does, he needs to move quickly from here on out. Because the game is so unfinished, he is going to lose his audience quickly to other games. This wasn't an issue before because it wasn't as popular, now that it's popular a stop watch until the copy cats hit the market has started.