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The app is now called Navmii. If you have an iPhone, simply download this free app which works as a sat nav.
MSE Darren tried this a couple of years ago and his view was "it works well as a sat nav, though some of the maps aren't 100% up to date. If you're doing a cross country trip it should get you there, but for accurate traversing of small road you may have some problems. Still if you don't have a sat nav, and don't need one, but would like one for back up - its a cracker for the price (ie free!)"
Also, the iTunes app page says "No data connection is required for downloading maps".
Thanks to revpeachy for the original post.