After fiddling a bit, here is a shell script using mencoder, that takes an .flv file and converts it to avi, (with mpg compression)

#/bin/sh
# output plays with ffplay or latest mplayer-1.0pre8
# takes an .flv file as arg1 and produces a basename.avi
mencoder $1 -vc ffflv -ovc lavc -lavcopts vcodec=mpeg4:vbitrate=1000:v
+hq -ofps 24 -oac mp3lame -lameopts cbr:br=96 -o `basename $1 .flv`.av
+i
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