Check out this neat use of QR codes (QR codes allow a person to take a picture with a camera phone and get sent a url) by Berlin based artist Sweza. Sweza places QR codes in the same location where previously street art sat, but sadly has been buffed. You can snap a photo of the QR code, and be sent to a page showing you the original art from that location.
It almost becomes a form of street art itself as the QR codes he places are hardly subtle. Although I think this actually might be half the point.
Pop over to Swezas blog for a little more info.