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.

It would be something more open, like Twitter.

And, FYI, most phone books actually are available to search on the internet. E.g., for the UK: http://www.thephonebook.bt.com/publisha.content/en/search/re...

Location sharing as in something similar to Facebook check-ins or Geotagging in Instagram?

Either, I'd be particularly interested to hear if you were open to one but not the other.