efficient terminal/console screencasting

[Andrew Pantyukhin]

A while ago I needed to screencast a terminal to a class of
students in real-time. It turned out to be quite straightforward
with screen(1) in multiuser mode.

Now I want to record screencasts and allow people to watch them
later. I'm looking at recordmydesktop, but the innefficiency
strikes me: I'm operating in text-only environment, there's no
need to record every frame as a flat bitmap. Whatever codec I
choose, the files will be huge and of low quality.

What I need is something like script(1) on steroids:
- accurately record the complete state of my terminal window at
  any given moment
- store it as a succession of updates, each represented in a
  text-based format (as opposed to pixel-based)
- record timings correctly
- allow for a method to record speech in the background
- allow for a method to play it all back in sync

Has anyone heard about any tool which would help me achieve at
least some of my requirements?