In the last while I’ve had to create several screencasts. After some experimentation I’ve found the following Gstreamer pipeline works well.

gst-launch-1.0 -e webmmux name=mux ! filesink location=test.webm \
  pulsesrc ! queue ! audiorate ! audioconvert ! vorbisenc ! queue ! mux.audio_0 \
  ximagesrc use-damage=false ! queue ! videorate ! videoconvert ! video/x-raw,framerate=5/1 ! vp8enc threads=2 ! queue ! mux.video_0

Couple of notes:

• use-damage=false is important if you are using a composited desktop (eg Gnome 3). It took a discussion with a dev on #gstreamer to figure this out.
• framerate=5/1 reduces the framerate to 5 per second. This greatly reduces the amount of video encoding required. My computer couldn’t keep up without setting this. The low framerate works quite well for a screecast and keeps the files small.