I just saw the ad here for the Tile locator chip for use with the IPhone. I don't know how well it works or it's practical limitations, but I remember someone posting recently about caching supplies in a forest and perhaps this would be helpful to get you to the right rock or log. Might also be able to add a twist to geocaching. I wonder if you could use one as a proof of being at a particular location.
I didn't see much detail on how it works, but I'm inferring/assuming that the tile and phone can communicate directly.
