How far do you generally travel with it? Google maps lets you take a 10mile radius and cache it. So maybe cache your area or the area you travel too so when you get closer the sat nav will sort it out without a data connection.
There is an app out there however, i found on on XDA but i can't recall it. It wasn't the greatest but i think there is more then one on there. Can't give you the name though, sorry about that.