Intriguing puzzle solved

Have you come across one of these?

We first encountered one in Basel, earlier in the year whilst standing outside the cathedral.

At the time, we weren't sure what it was/what to do/how it worked.  Until this evening, I still wasn't really sure, but a recent post on the Moo blog piqued my interest and curiosity again and finding that we have barcode scanners on our Nokia N95 phones, of course, I just had to find out more.

So there we are.  Another mystery solved.

See if you can decipher this green one - a small prize for the first person to find the link and leave me the answer in the comments! (Nokia N95 phones aren't essential!)