From the New York Times (link):
A visit to the construction site of the National Stadium in Beijing is as close as you get in the 21st century to seeing what it must have been like to put up the Great Wall of China. At one point, 7,000 workers were toiling on the stadium, dispatched in six-month stints from the countryside and organized like an army into squadrons. When I visited at the end of March, their number had diminished to a couple of thousand: the concrete had already been poured for the huge bowl that will seat 91,000 spectators at the 2008 Olympic Games, and the raising and welding of the steel columns and beams " tasks that require extra training and elbow room " were well under way. Cranes more than 300 feet tall hovered above, hoisting metal pieces as heavy as 350 tons to form a lattice of interwoven steel. Knowing that the nickname "bird's nest" has clicked with the Chinese public, I could imagine the enormous cranes as Godzilla-fied birds and the dangling curves of steel as worms being lowered for the chicks. The 24 main columns are gargantuan " 1,000 tons each, far more than the weight of those in a conventional stadium and spaced in what appears to be a random pattern. "Everyone thinks this is the most remarkable piece of architecture we have ever designed," the architect Jacques Herzog told me months before in Switzerland, where he lives. "To realize that project there is amazing." It defies expectations to see this avant-garde building rising in China, and yet, Herzog had remarked, "such a structure you couldn't do anywhere else."
See also a slideshow "Olympic countdown" from the New York Times (link).