We already have audio watermarking with Cineva which stops mutes playback on devices such as the PS3. Obviously the media player needs to implement detection of Cineva and then block playback (or audio in the case of Cineva which I believe causes the player the mute the audio). I very much doubt VideoLAN and MPC-HC will implement such things however I can see Windows Media Player and other "official" media players implementing it.
Cineva is interesting as from what I have read it can survive even pretty extreme audio changes so even the audio from a CAM copy with crappy mono sound will still carry the watermark from capture to encoded MP4 (or whatever you capture to). I have not seen any statistics on how successful it actually is at surviving in real world use though.
I don' think you can prevent analog camera's from recording video with watermarking. If you somehow prevent windows from playing back video based on some watermark scheme someone will just write a Linux program that strips / adds watermarks.
I guess a lot of the CAM pirates works in the cinema business. With watermarking the studios can find out exactly which auditorium it has been recorded in. I guess that will discourage a few pirates ...
