If you really plan on using a BB in a more harsh or outdoor environment a lot you might consider an Otterbox case. Frankly, I don't think many cellphones are as durable or, at least, as water tight as a real gps intended for outdoor use. BB, from what I read, are pretty water sensitive and have stickers to show if they have been exposed to moisture.
I am not sure about DATA coverage area - I have not used the geocache programs but if they rely on showing you an instant map of your area they may in fact need a data connection. But if they rely on just the gps and showing coordinates/arrows/distance, that can be done without any data. The gps gets its info from satellites over head and not cell towers and therefore, depending on the software, can show position lat/long without cell-tower help.