http://en.wikipedia.org/wiki/Augmented_reality- Augmented reality. Sound, videos or graphics can be used to enhance your sense of reality.
http://www.bookofjoe.com/2010/11/locative-art-in-the-new-york-times.html - Interesting article about smartphone app Layar and how 2 artists Mark Skwarek and Sander Veenhof added art to a MOMA exhibition.
http://www.traceencounters.org/ - 900 small pins were assigned to people and each stick remembered the pins it encountered.
Spectacle Island near Boston- Core Sample- GPS based interactive sound walk.
/www.youtube.com/watch?v=lMpAeuJd6YQ- Silver Island Bunker- locative media project for walking a trail. Virtual hiker...
I'm looking at doing a locative art piece for a public art installation at the new San Pedro Square in San Jose.