Exactly. The problem with location-based advertising etc. has always been that you need users to willingly give up their location. The genius behind Foursquare (and the other services like it) is that giving up your location is the whole point of the "game" - it's not a step required before you can get value from the service, it's the core action that the service rewards you for.

And really, this is the not-so-secret behind most privacy-threatening trends and technologies: most people will gladly give up their privacy for a [ sense of safety, comment from a friend, virtual badge ].

And if that is the value that most of the world places on their privacy, maybe there's nothing wrong with losing it.

Is your physical location really part of your privacy if you're in a public place?

I can understand that if you're at home on the toilet or something you'd want to keep that private. But when you're walking around the city, or going to a shop, those are public spaces. Therefore it's not private information that you would be divulging, as anyone in that location would have that same information.

Then if you didn't want people to know where you are online you would change the settings, or just not use the application. This is just another example of Clay Shirky's idea of Filter Failure.