Bokode (via) New take on the humble barcode from the MIT Media Lab—Bokodes are 3mm wide but can be read at a distance by a regular digital camera lens using out of focus photography, exploiting the bokeh effect. The way in which the Bokode is read allows both distance and relative angle to the camera to be derived, making it ideal for Augmented Reality systems.
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The main disadvantage seems to be that it's not flat (if I'm reading it right, the barcode + lens is almost 1cm thick), and to use a regular camera (without flash and polarising filter) you need a LED backlight behind the barcode.