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]