The local paper recently had an article on GeoVenture, a cell-phone
service that's currently being test in San Francisco. Basically, you
"point and click" your cellphone at anything you want information
about and it's delivered to your cellphone. You can get directions,
restaurant menus, movie times, etc. The inventor was asked to
speculate about future uses of the product.

"Or there's the treasure hunt. You leave a message on a marked spot -
say under a tree - and when the other player arrives at the spot,
GeoVector sends a message with more directions."
(Buffalo News [NY] Nov. 15, 2004)

Sounds like Letterboxing is going high tech to me!
-- Dagonell the Pirate P-)