I've read up on Zarqawi and have also read that not only is he in Iran, he may not just be a part of Al-Q, in concert with Bin Laden, but he's also been in COMPETITION with him in recruiting. His more public beheading of Nick Berg may go along those lines. I know they hold him responsible for nearly everything going back to the assassination of our diplomat in Jordan.
I agree different factions of a global movement (like worldwide communism before it began to fray and fracture in the 1950s and 1960s) may be in competition with each other (as in any organization, however loosely affiliated the parts might be, people at the top jockey for leadership).
But the words and target of the message were not what I would have expected if the idea is solely to produce a mass psychological effect through the gruesomeness of the means of execution. Would have been more framed at recruits. I am sure it didn't hurt recruitment, but it didn't seem that was the main purpose.
Zarqawi exposed himself deliberately by placing himself at a certain place and time. Different than an audio or videotape because you don't dump a body. And he is careful, so he knew. He managed to evade capture with the Marines surrounding Fallujah. What it reveals about his state of mind is that he either feels he (and his cell(s))is holding an extraordinarily weak hand (and he is coming unglued with rage), or he feels he is holding a uniquely strong hand. It was not a usual terrorist message to be sure.
Zarqawi is a mid-level leader making a play at being top dog, but he is not the ultimate threat. This is, as it has been from the start, state directed.