Cool idea! Could you cache the returned values, so that in case one doesn't have an internet connection, but is again in a place that he/she has been before, the information can be returned immediately? (not having looked at the data that is sent, this might not be possible, depending on how fine-grained the input data is)

It would be great for this application to create an "offline positioning database" based on all the cell information. Also, can't you just record all the cellIDs while a user is running around in offline mode, then (if the user wants to) geocode all these IDs when the user is online the next time and save all this in the database for later retrieval?