ok first off how you linking it to the map? you actually using lat and long of where they are? if you are I would use one of the extra fields in Threads for zipcode and then have a database for this that uses that field from the users table to link to a lat and long in the map database and then use that lat and long to display it on the map then whenever the user changes his zipcode it will change to there and if the zipcode isn't in the map database it won't display..
