Yeah, I expect tunnels or big buildings (such as wal-mart, target, etc..) would also cause problems, but thats not a big deal.
Who uses a GPS to navigate walmart? lol
All jokes aside, usually it pre-loads enough information to get you through any "bad or low signal" areas so most of the time you won't even notice it.
*blushes* umm.... you mean you guys DONT need gps in Wal-Mart? Some of those things are pretty gargantuan...
+1 for arm holster btw.