These could be used for viral marketing. Flood an area with them (on billboards, shop windows etc), then people will become curious and will start talking about these mysterious "B" coins. Then, when someone works it out, the news will spread by word of mouth like wildfire.
This seems like a good use for QR code... that alone could pique the curiosity of people with smart phones, though, it would be nice if there were already smartphone clients.
I wonder how feasible a QR Code stencil would be?