If you're not a gamer, it is almost impossible to express the significance of this news in the videogame world. But let us try.
The average lifespan of a videogame console is roughly five years. In this time, oountless games are developed for these consoles, most in far less time. Sports games, for example, are commonly on an annual development cycle. Developers experiment with the new hardware, come up with ways to use the hardware, push it to its limits, and eventually find those limits, all within those five years, through the development of game after game after game. If a game actually takes five years to develop, it's seen as a torturously slow process, usually rife with speculation on whether the game will be made at all.
Duke Nukem Forever's development cycle is 13 years. It was first announced in 1997. That means that, over the course of its development, it has had to technologically revamp itself completely, twice, to account for two new console generations, as well as 13 years of advancement in personal computers. It is getting cheered not for the content of the game (about as raunchy and violent as you'd expect a product with roots firmly in the 90's to be), or through any belief that the game will be good. There's been next to no discussion of that. It is getting cheered simply for being a completed product that will be available in stores. It's like a last-place finisher at a marathon who got beat by a bunch of people that decided on a whim to run the route three hours after it started.
In Duke Nukem Forever's case, the marathoner was found laying drunk on the curbside at the 18-mile mark, loaded into a taxi, and driven to the finish line. Original publisher 3D Realms originally planned to release it in the summer of 1998, and then spent the next decade-plus goofing off and having Nerf gun battles. Occasionally, every few years, the slightest, smallest, most pathetically puny piece of content would be released to the public that only served to underscore just how little was actually going on and provide even more comedy fodder.
In September 2010, 3D Realms ceased to exist. Another developer, Gearbox, took over the license and, unlike 3D Realms, decided they'd actually like to ship a product. On May 3, fingers crossed, we'll finally have one.