Greg Sterling reported at Search Engine Land a couple weeks ago about a Google Maps Sticker Spotted In London. Loren Baker also referenced this noting that no sightings of this sticker have been found in the US.
I finally found a thread with official confirmation from Google on this.
A Google Groups thread has Maps Guide Jen saying:
The stickers are part of a marketing campaign that Google did in the UK. There currently aren't any available for Google Maps in the US. I'll contact you individually with more information!
So I wonder if you comment in the thread, and if your in the US, can you get yourself a sticker. I will try.
Forum discussion at Google Groups.