The New York Times has an interesting article on the science of glass, profiling the newly-opened attraction at the Sears Tower, "The Ledge." Only three panes of glass separate you from the street some 1300 feet below.
Needless to say, a lot of thought, engineering, and safety precautions went into building this attraction. The article has a nice graphic illustrating the engineering that went into building the glass box.
If anyone has been out on the Ledge, I'd be interested to hear about your experience.
It's worth noting that the Sears Tower Skydeck is part of the Chicago Citypass program, which bundles dicsounted admission to four of Chicago's best museums (Museum and Science and Industry, Adler Planetarium, Shedd Aquarium, and the Field Museum) with a visit to the Hancock Observatory or the Sears Skydeck.