Originally Posted By: drakino
Google Maps Navigation (not currently available on the iPad or iPhone) will cache the route to prevent issues if the data connection is lost along the way. Text to speech also works fine when offline. The routing part I'm not certain, but it't probably what Bitt said, just good enough to get you back to the master route, and would fall apart if you deviate off course a ways.

What Tom said. It does cache the instruction for your route. If you wander off completely however, it won't know how to get you back to your destination without reconnecting since it doesn't store maps locally and therefore can't reroute you.