I like the idea of QR codes, encoding URLs and placing them on real world objects, but the QR codes themselves are kind of ugly. To make them less obvious I thought I could spray QR codes on to an object with an infrared reflective paint and shine infrared light on the QR codes, since most cameras, for instance the camera in my G1 phone, pick up infrared that our eyes do not.
In looking for this paint I've found that it comes up a lot in relation to the military for things like paint markers that are visible at night with proper equipment, and paint that absorbs IR light to make vehicles less obvious to night vision goggles. Even though the first reflects infrared light and the second absorbs it websites end up refering to both as infrared paint which made it difficult to search.
Additionally I found links to some other geeky infrared projects: