In reply to neilus:
> Do you think this would save valuable battery life?
Well, it will stop your phone needing to access the internet to fetch tiles, so, yes, it should reduce the power use. And give you a much better response when scrolling the map than trying to rely on 2G or 2.5G data (you're unlikely to get 3G away from towns).
As wilkesley says, it will allow you to put the phone in flight mode, therefore turning off the RF subsystem. Although if your phone is an iPhone, you may find that this kills the GPS receiver, too... Certainly, turn off wifi and bluetooth if you're not using remote sensors that need them.
If you have an Android phone, look at the detailed power usage to see what's using it (and recommendations for saving power). The display will probably feature quite highly if it's turned on a lot.