] Mie's cellphone can tag each photo she takes with the
] latitude/longitude coordinates. Last night she tried
] sending a geotagged jpeg for the first time, so this
] morning I started researching how to get the coordinates
] out of the jpeg. It turns out that the data is stored in
] EXIF headers.

Eventually you'll leave notes around in the real world, which your friends will be able to access at that location when they are there. Coming soon, actually... This is all very easy to do today, its just a matter of access and cost.