I just filmed a couple of clips to edit them later in iMovie. Surprisingly, the MPG files generated by my (consumer, low end) camera were not importable in iMovie.

Armed with ffmpeg $ brew install ffmpeg if you don't have it already installed) and bash, it wasn't that difficult to solve the problem, once I figured out the particular incantation that ffmpeg required. Therefore, to hopefully save someone else time if he/she has the same need, here it is:

for x in *.MPG; do
    ffmpeg -i $x -strict experimental -f mp4 \
           -vcodec libx264 -acodec aac \
           -ab 160000 -ac 2 -preset slow \
           -crf 22 ${x/.MPG/.mp4};
done

And voila, all the *.MPG files in the current directory will be converted to *.mp4 files, importable in iMovie.