The simplest explanation is that the guy was a US contractor who was killed - not an unusual event during a war - and that the US spinmeisters decided to use his death to detract the US public opinion from the deeds of the US mercenary forces - again business as usual.
The links that Kelley provided strongly suggest that the beheading took place post mortem. It is quite conceivable that someone in this administration decided to stage the gruesome spectacle to divert attention from the prisoner abuse scandal. All they need to do is getting a body from the morgue pay some of the Iraqi thugs on their payroll to behead it and then arrange the video to be sent to a known outlet of Islamist propaganda.
A similar stunt was pulled by the British intelligence during World War II who staged a plane crash in the coastal waters of Spain - the wreck contained a body of a "courier" (in real life, a civilian who died of pneumonia) with important "war plans" of the British invasion of Greece (completely fake, to divert German attention from the planned invasion of Sicily). The British intelligence counted on Spanish pro-Nazi sympathies to let German agents to have a quick look at the documents, before the body was returned to the British, and they were right. http://africanhistory.about.com/library/prm/blmanwhoneverwas1.htm
Wojtek