Since today one of the main features of a geocaching app is ready: offline maps!
You can create offline maps with programs like Mobile Atlas Creator (MOBAC) from different mapping providers, including OpenStreetMap. This maps you can transfer with iTunes to Looking4Cache. So there is no need for a 3G connection while caching.
Also it should be possible to use a iPod touch (from 2nd generation, because of gps via bluetooth) to find the caches.