A message posted on the 3D Realms website says, "Last Saturday we had our annual company Christmas party. It was a lot of fun as usual but it featured one special surprise. It turns out that several people had been secretly working late nights and into the wee hours of the morning preparing a special video for those at the party. They created a short teaser for Duke Nukem Forever."
"After seeing the teaser we thought it was something we should share with all of you and while it's just a teaser, rest assured more is coming. Tomorrow, Wednesday the 19th, around noon CST, we will release the first teaser trailer from Duke Nukem Forever. To tide you over until then, there has been a screen shot taken from the teaser and posted in our forums. Check it out here."
"Thank you for being fans of the game and for your continued patience."
An update to the message meant to "manage expectations" was added later by George Broussard, which said, "This is a teaser. It's not a full-blown trailer like the 2001 trailer (but something like that is coming.) I tried to be clear about that in the message board post, so just bear in that it's a teaser."
A link to the 3D Realms forum post on the topic is available here, although the server is experiencing difficulties with the increased traffic.