Via email, Valve mentioned that it was developing a new demo type format — for Team Fortress 2 or perhaps all Steam Games. This comes as players have been breaking demo files after downloading new updates.
Tell us about that Robin Walker! The Valve developer explains: "It's getting close to being releasable in a beta form."
The basic idea is that after players die or finish a round you are particularly proud of, you can ask the server to send you a demo of it. "The server then sends you a demo that it recorded, which has the complete game state instead of just what your client saw, and it's in a format that's robust and won't break as easily when we update the game," Walker adds. "In addition, the format has a some side benefits, like supporting full backwards and forwards time scrubbing."
Robin Walker interview [TFPortal]