Adding the x11grab plugin to ffmpeg - This allows you to make linux screencasts. Sweet!

This worked:

ffmpeg -vcodec mpeg4 -b 1000 -r 10 -g 300 -vd x11:0,0 -s 1024x768 test.avi

Another one:

ffmpeg -g 64 -r 8 -s 1024x768 -aspect 4:3 -vcodec mpeg4 -ar 16000 -vd x11:0,0 -qscale 1 ~/test.avi