I've seen a number of websites and journalists speculating that Berg's death isn't exactly what it was made to look like.
I haven't watched the video so I can't weigh in on that. But I've read a number of the websites that bring up a lot of unanswered questions, and if what they're saying is legitimate, it does seem pretty shady.
I'm not big on conspiracy theories, so I think I'm going to have to reject the idea that Berg's death was faked by the CIA. Personally I feel that if the CIA was going to do something like this, they would have done a better job of it. But I don't think I can take the tape at face value either. Even CNN, a legitimate news network and not just some online conspiracy nut with a blog, had a expert who studied Middle Eastern cultures and languages who said that the man in the video did not sound like al-Zarqawi (he had an accent that was not Jordanian). There's a lot pointing towards the killer not being al-Zarqawi . . . but if so, then who was it?
There's also quite a bit that points towards the fact that Berg was actually dead at the time of his beheading, and it was faked to look like he was still alive.
Here's a website that covers some of the major discrepancies. He does point fingers at the US for Berg's death, but if you ignore that, it still brings up some questions:
http://www.kuro5hin.org/story/2004/5/15/22827/0477