Here's an e-mail I got from Yahn Bernier a while back.
"Quintin does most of the moviemaking. Copying him in case I missed a step below.
If you want to record a fixed framerate playback movie with a properly synced .wav track, we generally do this (from the console -- we don't currently have a UI for doing moviemaking)):
host_framerate 30 (forces fixed framerate for playback)
playdemo xxx (starts the demo playback)
startmovie css (starts dumping demo frames to file and writing the .wav)
You do have to have a .dem file, obviously. Not sure whether there are any existing issues with .dem files in the CS:S beta, but if so, please let us know. They should work (or they did last time I checked).
Additionally, there is a demo playback UI available by pressing Shift-F1 when in the game, it's currently used internally only at this point so no promises that it doesn't have any bugs.
Yahn"
go to steam\steamapps\e-mail\half-life 2 and you'll find the screenshots there some where.
ps: The wav file was corrupted everytime I did the preceding method with CS:Source, haven't tried it yet with "final" version of CSS nor HL2.