Augmenting reality using the GPS&Compass combo has become very popular in 2009, and has decent accuracy when used to augment distant big objects such as buildings. However, it won’t be terribly useful for augmenting specific floors within a building.
Japanese Qosmo and Teradadesign Architects to the rescue. By covering the facade of Tokyo’s N Building with giant QR codes, they were able to display store information and even tweets, linked to their source from within the building. It’s cooler than my lame description, check out the video: