A Physical Wikipedia
The essential ingredients are mobile devices, a database such as Wikipedia, and some kind of technology to link the two together. That link might be a low-tech construct such as Yellow Arrow, or something like ShotCode, a technology that allows camera phones to "read" bar codes that link back to related URLs.
Mobile Weblog has an example of how all this might work:
You're in London and are standing in a pleasant, sunny street in Camden Town. City life is going on around you and you fancy the idea of knowing a little more about where you are right now.
Using your phone, as if it was a PC mouse, you uncover snippets of information from the world around you. You click on an old house in the road and a wealth of digital information comes onto your phone screen. Some contain video and audio links.
You learn that the house is on the site of one lived in by Charles Dickens' wife after their separation. You're interested in Dickens so you poll the area and find that there's actually a tour of Dickens' Camden Town that afternoon.
Out of curiosity, you look up how much this kind of house would be worth, what local rates and taxes are. And you read a review of a local citizen's view of schools in the area.
Moving on you see a tree, which looks unusual and casually click on it to reveal its genus. Then you click on car you like the look of, to find out how much it would cost second hand (2003 model), where you might be able to find one and what the gas consumption is like.
Sources: Mobile Weblog, unmediated