Viewing a V4L webcam with mplayer…
This took me a few minutes, but I figured it out, and archived it here so I wouldn’t need to spend that time again.
mplayer -cache 128 -tv driver=v4l:width=640:height=480:outfmt=i420 -vc rawi420 -vo xv tv://
It works.

[tags]My Projects,Webcam,Mplayer[/tags]
Addendum: You can specify which video device by adding it to the tv options:
mplayer -cache 128 -tv driver=v4l:width=640:height=480:outfmt=i420:device=/dev/video1 -vc rawi420 -vo xv tv://