I'm fairly sure netflix allows you to do this already.

Not if I don't know my wife's login credentials and not if I'm watching a home video or a DVD that I've ripped.

Wouldn't you just sign into the same netflix account on both computers? The only other way I can imagine to do it would be to pair accounts together somehow, but that could be awkward if you and your wife are at different points of watching the same thing since you would lose each others places.

I think Plex media server might do some of that for normal video formats, though I can't see Google and the MPAA being excited about building software to make playing ripped DVDs easier..

in those scenarios you wouldn't be able to pick up on another device either.