I have been struggling with this all day but here is the command line that finally made something that had audio and video that worked.  The video is OK and the audio a bit hissy, so more work to be done there.

ffmpeg.exe -i california.mp4 -vcodec mjpeg -acodec pcm_s16le -ac 2 -ar 44100 california.avi

The video with this command line is terribly blocky and pixelated, so more needs to be done to get it totally right.  My 3 minute video is 130MB with that setup.  I can improved the quality by using -qscale 1 (see below) but the size balloons to over 400MB.  Since the Wii can only take 2GB cards that is a slight problem.  A trade off I guess.  I will have a play*.

ffmpeg.exe -i california.mp4 -vcodec mjpeg -qscale 1 -acodec pcm_s16le -ac 2 -ar 44100 california.avi

Hopefully this will be indexed by google and can help others.

* results of the play – my movie is 3:47 long and as mp4 is 42MB
-qscale 1 and -qscale 2 give identical results at 474MB
-qscale 4 @ 330MB and pretty much the same as 1 and 2
-qscale 6 @ 271MB and still pretty good
-qscale 8 @ 233MB and quality has deteriorated slightly

http://www.kilobitspersecond.com/2007/05/24/ffmpeg-quality-comparison/ was interesting once I had found qscale as a search term.