ZoneTag is a little prototype that was developed by Yahoo! Research Berkeley. A look under the hood shows that it's built on top of a couple of interesting APIs that we're opening up. Anything that ZoneTag can do, you could do as well.
Want to know the location of a cell tower? Update the location of a cell tower? Get a list of suggested tags for a location? Open your web service to any mobile ZoneTag user without having to do mobile programming? It's all possible! Have a look at the documentation.