Too many coincedences.
Berg knew Moussaousi, let him use him computer for email.
Most likely, Berg was a convert to Islam.
Probably went to Iraq to make contact with AQ or other terrorist operatives.
Was arrested by Iraqi police and questioned by US Military.
When he got free, he met his contacts and told them what happened. At that point, he was a HUGE liability/security risk for AQ, so they detained. When they needed someone to "make a point", they simply beheaded him on camera. Solved 2 problems at once.
"Most likely, Berg was a convert to Islam."
You know, I hadn't thought of that one. Note (from the film) the very Islamic (or ahmish?!?!?) looking beard.