To be honest, I thought the game's supposed existence was a vast conspiracy. More than a couple of times over the last decade I have gotten vague invitations to come see the game "someday," with it never happening. I figured this was probably the same invitation other developers in the area had gotten, and were claiming that vague invitation as having actually seen the game.
Nothing of the game has been made public for years until that Duke Nukem Forever teaser was released last December.
However, that all changed recently. For a good half hour Shacknews witnessed several different DNF gameplay scenarios, mechanics, and environments being demonstrated.
We actually got to see the truth of Duke curator George Broussard's many past claims, including environmental puzzles and interactivity, a host of finished weapons, the existence of an in-game forklift, and plenty of heads and arms being blown off.
We both confidently agree that after seeing this demonstration, Duke Nukem Forever is looking great, and will easily stand apart from the crowd in both visual and gameplay styles.
Update: George Broussard of 3D Realms has made a brief comment in this story.
Shacknews can also confidently state that, contrary to previous reports, Duke 4 will not be arriving in 2003.
Art credit to Duke4.net.