Isn't this what BlockChalk [1] and many others have attempted before? How are you going to solve the chicken & egg problem? How are you more than a GPS centric twitter?

I'm genuinely interested. I actually created and submitted this exact type of application when Android first came out and had their app contest. Of course, the market has grown a bit since then. ;) I've since moved on but it always did seem like a good idea.

[1] http://techcrunch.com/2010/01/08/blockchalk-location/ (Deadpooled)