as far as I know, there's no such thing as an M4V file... technically. it's just a file extension that's used by some vendors instead of the MP4 you'd normally see.

So what you have is an MPEG4 file. If it's created for playback on an Apple device, it's probably MPEG4/AVC (=H.264) instead of the regular MPEG4/ASP.

N800/N810 does play such files. There are restrictions to bitrate and other encoding parameters, though, so if it doesn't play, it's not because it's "M4V", but because it happens to be not the right kind of M4V.

Depending on which platform you're running on your desktop, use any method described in the Wiki to make the file run.

(Oh, btw: on H.264 files, Nokia's Media Player usually does a better job than mplayer and other players using mplayer as their backend.)