Roger Ebert just added the French Film CACHE (2005) to his Great Movies section. And in it, Roger thinks he's found the smoking gun as to who was sending the mysterious videos. Here's my take on it.
Warning! Spoilers abound!



On IMDB forums, there has been much discussion on the shot that Roger mentions in his review, specified at 20:39 of the DVD. Some people have been focusing on this shot.

I've studied this shot over and over and can find no discernable aspect that seemed revelatory. Even the side mirror from Pierrot's side doesn't reveal anything upon closeup. Nothing is revealed from his bag. But take note that Georges's car is right at the spot where the film's initial recording shot is made. So the clue lies here.
Now note the time (20:37). This is not the shot which Roger is talking about. If we move 2 seconds further into the film...

In this shot where Georges's guest is recalling a story, you can see that part of his narration is the film's clue. There's only one mirror at the far end of the screen in the first shot. And that is on Pierrot's side.
The film's very subtitles tell us that Pierrot is the one who was sending the videos.
Addendum:
I just spoke with a friend who mentions that the scene in question might be this one:

As Roger states in his review, the film's completely "still" shots are the objective ones, while the ones that move (including the ones in "rewind" mode) are subjective. This one (we'll call shot #3), though this seems to belong to the former, I'm inclined to believe belongs to the latter, as it specifically refers to the spot where Georges's house is being recorded (the film's opening shot). It has to be a POV, but from who's?
The film tricks us (as it did me) with the succeeding shot of a boy with a bleeding mouth. If you watch carefully, the camera pans across the room to the bleeding boy by the window. This is not Georges's adult home, it's from his childhood home. The living room in this sequence is the same as the same sequence later in the film where Georges is leaving his mother's house. The boy I believe is Majid, from Georges's childhood memories.
Think about it. Shot #3 I believe is from Pierrot's POV, looking at the spot where he can record his videos. The shot involving the bleeding boy reveals why Georges must have wanted Majid to be taken away. As a boy, he must have discovered Majid bleeding, and being young, did not understand what his condition meant, leading to the film's disturbing revelations.
One thing that is clear about this movie, is that Haneke has complete understanding about how film narrative works and how ingrained audiences have become in its expectations of it. What an incredibly layered mystery this film is.



