This page has a counterpart, Theora, for Ogg Theora video. Many of the techniques on Theora can be used to also encode WebM.
MEncoder
mencoder -ovc lavc -oac lavc -of lavf -lavfopts format=webm -lavcopts acodec=vorbis:vcodec=libvpx -ffourcc VP80 input.avi -o output.webm
Confirmed working with r31918. More information available with this mailing list thread.
