Put it this way. Iphone have GPS SYSTEM but Don't have map like garmin(not sure the name) and others navigation. Iphone used GOOGLE map as map Which is equivalents to MapKing/others. So u are paying the Kb for google maps dl from 3G, edge,gprs.

You can use offline maps, but gonna take some work to do it. u need to save google maps as cache and transfer it though SSH, and also make sure don't accidentally deleted from iphone >.<

From my experience using GPS in Iphone, you need to ON "3G or EDGE or GPRS" but since i have a some cache on the maps (i not using any offline maps yet and iphone somehow got cache for the maps i view from google), it never eat my $$ . this maybe because the iphone gps is A-GPS, where it use the cell tower to locate where you are.

Basicly, the GPS u need on the 3G/EDGE/GPRS to locate your location, but not eat $$, as long u nvr load the maps.

It should be work same concept as offline maps installed.
PS: having exam, prolly will try offline maps system when i finish my exam and projects ^^

Ruix, refer to digi website, i believe they have the instructions.