Yeah, the Nick Berg video had a ton of faults and inconsistancies. I don't know about the later ones, probably they were authentic, but maybe whoever was doing them just got better at covering his tracks.
I think this proves how easy it is to fake a video. Now, when a headless body shows up in a river, no one is disputing whether the killing part was authentic, just who was doing it. If a kid with a video camera and some 5 dollar fake blood could slip by undetected, then how convincing do you think someone with real resources at their disposal coukd make it.