3D Realms has been shut down, it is now confirmed by a staff member. Development on Duke Nukem Forever has ceased, making the future of the title uncertain.
3D Realms was founded in 1987 under the name Apogee. The company popularized the "shareware" marketing and distribution model with popular PC games like Commander Keen, Cosmo's Cosmic Adventure, Wolfenstein 3D, and Duke Nukem. Some of those games were developed in-house, while others were distributed on behalf of other companies, like Id Software.
In 1994, Apogee decided to focus exclusively on games built using a 3D engine, and changed its name to 3D Realms. They produced popular games like Max Payne, Shadow Warrior, and Prey, but became known as much for a game they never released as for the games they did.
In 1997, the company announced it was beginning work on Duke Nukem Forever, a sequel to the popular Duke Nukem 3D. The 12 years since have seen the game identified as legendary vaporware, and many have openly speculated whether it would ever be complete, though occasional updates and game trailers have sparked rumors that it was close to completion. Amazon.co.uk listed the game available for pre-order, with a due date in December, but that was unconfirmed by 3D Realms.
With 3D Realms closing this week, the future of Duke Nukem Forever is again uncertain. Distribution rights remain with Take Two Interactive if a game is ever produced, but development funding has completely ceased.
Recently resurrected as a separate company, Apogee Software claims via Twitter that they are "officially not affected by the situation at 3DRealms" and that "development of the Duke Nukem Trilogy is continuing as planned." The Duke Nukem Trilogy is completely separate from Duke Nukem Forever, and is being produced under license from 3D Realms.
There has not yet been an official statement from the company. All information related to the closing so far has come from partners and (former) employee posts on web forums.