Would this be something like Google Latitude, where I share my present location (privately, limited to you, a select group of friends, Google, and the NSA; or openly, à la Foursquare or Twitter)?

Or are you envisioning a phone-book-like service where I post a more general "I live here" sort of thing?

I would not participate in such a service. It's arguably much more invasive than a phone book - for one, phone books are usually only available in that geographic vicinity, and are not readily searchable like an online service would be.