There are 2 apps that can be used with the iPhone/iPad/iPod Touch units.

iGeoKnife, this allows the database from GSAK to be loaded onto your device as well as:
Main Feature list:
* Import GPX files
* Make custom filters
* Sort by database column
* Search on name, cache-code or owner
* Navigate to a cache on map or using the build in compass
* All additional waypoints will be shown on map when you navigate to a cache
* Create custom waypoints, e.g. when solving a multicache
* Use the map to browse all your caches
* See the corrected coordinates if any
* View the cachedescription, logs, hints, waypoints and user notes.
* Log your finds

$3.99 from the App Store.

There is also Geosphere, this also works on the iPhone/iPad/iPod Touch and includes the following features:
-Import GPX or LOC files
-Import ZIP archives containing multiple GPX files.
-Import custom GSAK fields like Corrected Coordinate.
-Display a compass showing direction to cache.
-Display description, hint, past logs, inventory.
-Store description images for use offline.
-For a given waypoint, show all other related waypoints.
-Organize caches into the groups of your choosing.
-Filter by group, type, size, difficulty, terrain, found, distance, ...
-Sort filtered search results by distance, waypoint, or name.
-Highlight caches so they get your attention.
-Ignore caches that you don't want to do right now.
-Create field notes which can be uploaded to make log entries.
-Create custom bookmarks for the built-in browser.
-View details of a target cache as well as another cache.
-If you update a coordinate, original location is saved.
-Can manually enter your location to locate nearby caches.

5.49 from the App Store