I'm jumping in late and not reading the 7 pages of debate preceding this, so forgive me for any reiteration.
First off, in regards to the video a grainy still and some uneducated opinions do not a solid body of evidence make. Yes, several of the eyewitnesses have commented that it sounded like a missle. Have any of those witnesses actually heard a missle, or a 757 at that range? And as to the size, it's very difficult to make an accurate guess at the size of a plane going 350 mph, especially when it's juxtaposed against the sky, as you have no frame of reference and therefore can't tell if the planes 100 yards away or 1000. These aircraft are incredibly loud and due to that may have lead some of the witnesses to believe that the plane was much nearer than it actually was.
Next, forensic analysis of the crash (lifted from interviews of private firms in Popular Mechanics, not done by me). First, the windows. These aren't bullet proof windows, they're blast proof. This is a much stronger type of window designed to withstand the force of a bomb going off next to the building. The windows in the area directly struck by the plane were destroyed, but windows on the upper level were well able to handle the force caused by the hit.
Next, the hole. This was not made by a 757, nor was it made by the nose cone. In actual fact, the hole was made by a landing strut; this is backed up by images of the area between the rings where a rim with the NTSB mandated double bead design for passenger aircraft was found.
The plane, a comparatively light passenger aircraft not designed to withstand any collision more rigourous than birdstrike, struck the wall of the pentagon, a structure designed to withstand immense amounts of force. The plane was travelling at approximately 350 mph, which translates to a lot of kinetic energy; excuse me for not doing the calculations. This energy caused the aircraft to disintegrate; unlike a crash into the ground where the plane may divert some of it's kinetic energy by skidding along the earth (digging up wide furrows like the ones shown in the movie), this aircraft struck the Pentagon with nearly all of it's forward momentum intact. That momentum wasn't sufficient to cause the plane, signifigantly less structurally sound of the two objects to punch through the Pentagons walls, although the solid steel landing strut was able to sufficiently concentrate the energy behind it to puncture several rings. The rest of the energy, having nowhere to go, tore the plane apart. There was a lot of wreckage on site, although none is clearly visible in the images on the movie. Observe:
This image shows debris from the plane scattered across the Pentagon lawn, the nearest piece clearly bearing the airline markings.