This looks pretty rad - modulo the normal qualm of sharing all your location data with google, of course. I've been fiddling around with a lot of ideas for geolocated X on mobile (local chat, virtual graffiti, flashmob coordinators, etc) and the infrastructure to decide who is where is a bit heavy. Farming that to google at least for the MVP stage seems like a reasonable decision. Then, if we decide that google's tracking is a significant barrier to user adoption, we can roll our own once we're happy with how it's going.

This reminds me of Streetpass on the 3DS, one of my favorite featueres of that Nintendo handheld. This opens up all sorts of awesome possibilities for mobile games if it works as Streetpass currently works.