This may be a little off topic, but may be of interest to owners of the Western Digital TV Media Player. I have the "Live" version of this player, which can be networked wirelessly or through an ethernet connection.
I found that .m4v video files would play on the WD Media Player, but would not _fast forward_. The files were mostly encoded as H264/AAC. So I simply _changed the file name_ to .mov (which is only a container, not a codec) and found that the video could then be fast forwarded. No re-encoding was necessary.
What I haven't figured out yet is how to get the WD Media Player to recognise my iMovie chapters, which I always insert for DVDs. Anyway, at least I can now fast forward!
Sorry to hijack this thread, but my experience with the Media Player may be of interest to other users looking for a simple way to convert .m4v files to .mov - however, it may not work in all situations.
John