QR codes seem to be quite interesting.

A very similar conecpt with a different encoding method: http://www.semacode.org/
(Free reader software for Java-enabled phones, tagging, and more)

Haven't seen any of those in the wild yet. But if the tagging community (e.g. http://en.semapedia.org/) will grow and the software will be widely available, then this might be a cheap and very useful technical method for location based info systems.