"The video is of poor quality, and its time stamp seems to show an 11-hour lapse between when the assailants finish their statement and push Berg down, to when they behead him"

I have studied the video of the alleged beheading, this evening, and I would like to give you my own assessment of what I noticed about it.
The video starts at 13.26:24

13.26:24 - :27 Nicholas Berg sitting on a chair; camera angle from right. with hands untied "My name is Nick Berg, my father's name is Michael
2.18:33 - :43 Here he speaks straight to camera "... my mother's name is Susanne..." He then tells us his family members' names, and where he is from.
There is an obvious cut, here, as the next frame has Mr. Berg sitting on the floor, hands tied behind his back, and feet bound
2.40:03 - 2.44:12 - Speech from the terrorist in the centre of the video wearing black mask, and Mr. Berg is pushed to the floor.
13.45:47 - Video resumes, but the picture is very blurred, and the time on the video jumps eight seconds at 13.45:52 to 13.46:00
13.45:52 - 13.46:33 The 'Beheading'
13.47:47 -:53 Displaying the head of Mr. Berg
2.46:18 - :22 Displaying the head
13.48:38 - :49 A 'pan' shot circling above his head, lying on what appears to be his torso

There seems to be a synchronisation problem with sound to video; the speaker 'coughs' at one stage, but he doesn't 'cover his mouth' until a good 5 seconds after it.
The screams of this poor unfortunate man, however, are audible 7 seconds before he is pushed to the floor, and the person who made the speech is the person who is seen producing the knife and beheading Mr. Berg.
When the video cuts to 13.47:47, the person who is standing next the speaker, a tall man with a white mask, is seen holding the knife, and raising the head. A different camera angle at 2.46:18 shows the same person (apparently) displaying Mr Berg's head. I say 'apparently' because the camera shoots him with the wall as a backdrop, and not at all out of proportion to the picture. The person in the white mask who is standing next to the speaker, while he is making the speech, is the tallest of the people there, is of a different build, and the top of his head is out of camera shot. It 'looks' like a different person in the final frames of the video.

All of which may mean a lot... or nothing. The beheading seems real enough though, I am very sorry to say.

I think the video was made using two different cameras, however, and the 11 hr discrepancy that MTed refers to may be a case of the two cameras having different display settings; ie one having a 24 hr display (13.26), and the other having a 'AM/PM' display (2.40)

A one hour time difference between the where the owners of these cameras last synchronised their equipment, could account for that discrepancy.

Either way, it was an entirely academic exercise.
The fact remains that Mr Nicholas Berg spent his last moments in incredible fear and shock, and my thoughts are very much with his family this evening.

Jim