Last year Slate published an article called "What You Think You Know About Sept. 11...But Don't", including #2:

2. The misconception: We know how the hijackers seized the planes. Within days of Sept. 11, Americans believed they knew how the planes were grabbed: Terrorists had taken control by stabbing pilots, passengers, and flight attendants with box cutters and knives.

What's wrong with the story: It's incomplete and misleading. We don't really know what happened on the planes. The cockpit voice recorder survived neither New York crash and was damaged beyond salvage in the Pentagon crash. The Flight 93 voice recorder doesn't start until several minutes after the hijackers took the plane. What little we know about tactics and weapons comes from phones calls made by passengers and flight attendants. As Edward Jay Epstein has pointed out, the evidence is incredibly paltry. No one on United Flight 175, which crashed into the World Trade Center, reported anything about weapons or tactics. One flight attendant on American Flight 11, which also crashed into the World Trade Center, said she was disabled by a chemical spray, while another flight attendant said a passenger was stabbed or shot. On the Pentagon plane, American Flight 77, Barbara Olson reported hijackers carrying knives and box cutters but did not describe how they took the cockpit. And on United Flight 93, passengers reported knives but also a hijacker threatening to explode a bomb. The box cutter-knives story isn't demonstrably false, but it serves to divert attention from the other weapons and to mask the fact that we don't have any idea how the hijackings happened.

Well, now the 9/11 Commission Report is out, and we've got a pretty damned good idea how they took the planes. From the commission's section on Flight 11:

At the same time or shortly thereafter, Atta—the only terrorist on board trained to fly a jet—would have moved to the cockpit from his business-class
seat, possibly accompanied by Omari.As this was happening, passenger Daniel Lewin, who was seated in the row just behind Atta and Omari,was stabbed by
one of the hijackers—probably Satam al Suqami, who was seated directly behind Lewin. Lewin had served four years as an officer in the Israeli military.
He may have made an attempt to stop the hijackers in front of him, not realizing that another was sitting behind him.
The hijackers quickly gained control and sprayed Mace, pepper spray, or some other irritant in the first-class cabin, in order to force the passengers and
flight attendants toward the rear of the plane.They claimed they had a bomb.
About five minutes after the hijacking began, Betty Ong contacted the American Airlines Southeastern Reservations Office in Cary, North Carolina, via an AT&T airphone to report an emergency aboard the flight.This was the first of several occasions on 9/11 when flight attendants took action outside the scope of their training, which emphasized that in a hijacking, they were to communicate with the cockpit crew.The emergency call lasted approximately 25 minutes, as Ong calmly and professionally relayed information about events taking place aboard the airplane to authorities on the ground.
At 8:19, Ong reported:“The cockpit is not answering, somebody’s stabbed in business class—and I think there’s Mace—that we can’t breathe—I don’t know, I think we’re getting hijacked.” She then told of the stabbings of the two flight attendants.

From the description of Flight 175:

The hijackers attacked sometime between 8:42 and 8:46.They used knives (as reported by two passengers and a flight attendant), Mace (reported by one passenger), and the threat of a bomb (reported by the same passenger).They stabbed members of the flight crew (reported by a flight attendant and one passenger).
Both pilots had been killed (reported by one flight attendant).The eyewitness accounts came from calls made from the rear of the plane, from passengers originally seated further forward in the cabin, a sign that passengers and perhaps crew had been moved to the back of the aircraft. Given similarities to American 11 in hijacker seating and in eyewitness reports of tactics and weapons, as well as the contact between the presumed team leaders, Atta and Shehhi,we believe the tactics were similar on both flights.

From the description of Flight 77:

At some point between 9:16 and 9:26, Barbara Olson called her husband, Ted Olson, the solicitor general of the United States. She reported that the flight had been hijacked, and the hijackers had knives and box cutters.

And from the description of Flight 93:

At least ten passengers and two crew members shared vital information with family, friends, colleagues, or others on the ground. All understood the plane had been hijacked. They said the hijackers wielded knives and claimed to have a bomb.The hijackers were wearing red bandanas, and they forced the passengers to the back of the aircraft.
Callers reported that a passenger had been stabbed and that two people were lying on the floor of the cabin, injured or dead—possibly the captain and first officer. One caller reported that a flight attendant had been killed.

The Slate writer says that "Within days of Sept. 11, Americans believed they knew how the planes were grabbed: Terrorists had taken control by stabbing pilots, passengers, and flight attendants with box cutters and knives." and that this was a misconception.

Does it still sound like one? Sounds pretty damned accurate to me.