I just watched this, and really enjoyed it. Let's start with the ending. I'm assuming you noticed that Majid's son and Pierrot meet, talk and then go their separate ways. This could mean one of two things:
1. Pierrot and Majid's son were in on it together.
2. Majid's son is trying to reconcile what happened to his father by reaching out to Pierrot.
Personally, I'm inclined to believe the former. Having said that, if the second is what happened, meaning Pierrot and Majid's son didn't send the tapes, then who sent them? I read on Wikipedia that they could be metafictional, and that Haneke, the film's director, "sent" them to the Laurents, which is an interesting theory. This seems to be supported by the fact that the drawings foreshadow Majid's suicide.
As well, I read in some reviews to pay attention to the swim meet. I did, but couldn't find anything worthwhile, other than a teenager with a camcorder, which seems to be a dubious clue at best.
All in all, I thought that this was a taut, intelligent, provocative film. It works well on many levels - an examination of guilt in contemporary France and the West, a criticism of liberal intellectuals choosing to ignore pressing issues, a suspensful Hitchcockian thriller and, possibly, metafictional mindbender.