It appears that different versions of ffmpeg output information in slightly different ways. The ffmpeg from macports will fail to encode any videos because of this. There is a simple patch you can find at http://github.com/mhs/rvideo/tree/ffmpegfix

We didn't notice you had pushed to github originally so we pushed out a mhs-rvideo gem. If you can include this patch we'll take our gem down and move back to your rvideo gem. Thanks,