It makes sense that you're getting google navigation to work even when your data connection dries up. Google navigation downloads all of the relevant information for your route when you first put the route in. The problem is getting back home....
When you're at your house (and presumably have internet), google nav will download everything you need to get to your destination and will not lose it if you never have an internet connection again. When you get to your destination and stop navigation though, you will need a new internet connection to be able to navigate back home. This is particularly problematic if you end up in the middle of nowhere.
Android is the obvious future, but it is a rather large problem if there are no apps (currently working) that can perform truly offline navigation.