I created a stamp of my mobile blog QR Code (size: 2cm x 2cm).The results are amazing!
Even a stamped QR Code like this is still readable with the Kaywa Reader.

Jabberwocky

While today’s mobile communication tools readily connect us to friends and known acquaintances, we lack mobile devices to explore and play with our subtle, yet important, connections to strangers and the unknown – especially the Familiar Strangers whom we regularly encounter.

Now your mobile phone can expand your sense of involvement, belonging, and identity across your daily urban landscape in dramatically new ways.

From info page:

The principle metaphors of Jabberwockies are “digital scents” and “digital tagging”. As individuals traverse an urban landscape, they simply infuse their path with a unique and detectable digital redolence. Similarly, fixed places/objects can also emit unique “scents” once they are “digitally tagged” with small wireless tags called iMotes.

Why not simply use QR Codes;)