Geotagging/Flickr

I have acquaintances who are fans of geocaching. For those of you unfamiliar with the term, there are lists of markers placed around the world that people seek to find using GPS coordinates and clues. Sometimes you just locate the marker; sometimes you leave something small inside a capsule. On Flickr, there is a link to geotagged photos, which appear to be another form or enhancement of the game. There is substantial additional information on Wikipedia about geotagging and Flickr's role plus numerous links. I think this could be fun to pursue while traveling.