I think this is one of the most creative ways I've seen to bring patrons into your library. Participate in a Geocaching game!
Spring Township Library right here in my home state of PA is doing just that! They created a fake book and drilled a hole in it to hold the cylinder. They even gave it a call number. Take a look:
Be sure to check out the cache page and read the first finder's web update. Have any other libraries done this? Don't you think it's a great idea?