I can vouch for that too, and I think it'll probably be another 4 or 5 years before any other language starts to make significant inroads into game code. I've worked with people in games who simply don't know any other languages.
(You can be a perfectly solid C++ programmer without needing to know anything about, say, Ruby or Scheme, but in my opinion, the more you learn about programming in general, the better programmer you will be.)