It's been over 10 years since 3D Realms announced Duke Nukem Forever. So long ago, in fact, that when it was announced, I couldn't legally drink!
The teaser was made by some 3D Realms employees to surprise the other employees at their Christmas party. Management thought the teaser so great that they went ahead and released it to the world. Thanks, I guess.
For a little history on the game, and everything that has happened in the game-o-sphere/world since, you should check out this break down.
For the 2001 official trailer (a mere 6 years ago), go here. You know, that would've been a great game in '01 and they could've put out another one by now!