Got a video camera with a nightvision (infrared) setting? Grab your iPhone and take a look at the left-rear of the phone, as fskj85 of Austrialian Whirlpool did, and you'll see the snazzy "Data Matrix Code" underneath the body of the device. Wazzat, you ask? It's essentially a two-dimensional bar code (many Nokia phones are able to photograph these to get links to downloads, for example). Apparently the plastic in that section is transparent to infrared light, allowing you to see the matrix underneath. That's some secret-agent-design right there, folks, somebody nominate Jonathan Ives as the next James Bond.
Engadget Mobile, where we first saw the story, posits that the matrix likely encodes the IMEI and the Serial for the iPhone. That information is also printed in human-readable form on the SIM-card tray, but since that tray could technically be removed / swapped into another iPhone, it makes sense that Apple would find a way to get that data onto the iPhone itself.
That, or Apple is secretly tracking us with every camera we pass by.