The Duke is not dead. Yet. If all goes well--and really there's no telling how something will turn out when 3D Realms is behind the wheel--Duke Nukem will be making another appearance over Xbox Live Arcade. The official Xbox website has listed Duke Nukem: Manhattan Project as a release over the digital games service, citing 3D Realms as the developer.
Manhattan Project isn't a new game, nor is it a standard Duke romp. It's a 24-level, New York-set side-scroller which originally hit the PC in 2002 courtesy of Sunstorm Interactive (with a 3D Realms overhead). The game got a decent amount of critical buzz around the time of its launch, but from what I gather, it lacks something critical to the success of any title: flushable toilets. The description of it, however, does mention "explosive 3-D action," which might mean you can blow up toilets--a passable substitute for flushing. Either way, I'll actually be able to confirm this a bit later since the original title is still available for download on the PC.
The Xbox page doesn't list a price or a release date for its re-release. A good guess would be "soon" and "10 bucks," but that would be assuming 3D Realms was operating in a traditional, predictable manner.