[gitmo]

Several possibilities:

Berg was an American and was killed in retaliation for Abu Ghraib.
Berg was CIA or Israeli intelligence.
Berg was working with terrorists but somehow betrayed them. They used the situation to further inflame the prisoner abuse story.
Berg was working with terrorists / Saddam supporters and was executed by CIA or some other American group.
Berg was in the black market and crossed the wrong guys.
Berg's father promised his friends that his son would help their cause, but Berg refused.
Somebody else was killed.

[lelio]

Berg was working with terrorists but somehow betrayed them.

My leading possibility is "Berg was an enthusiatic lad and was interested in putting up cell towers and would work for anyone. He naively thought these people wanted to do some work and was captured."

What gets me about the still images from the video (I don't want to watch it) is that he looked really calm in the beginning. That makes me think that either he was in on some sort of scam (ie captured by fake terrorists and escaped) or he wanted to try and help the terrorists get someone released from prison. I sure as heck know if I was on camera with those nutballs I would be fighting for my life.

[justanotherday]

Which of the options you presented do you think is the real answer?