Well, the current state of geocaching apps for the iPhone is a bit thin on the ground at the moment. From what I can tell there are two contenders (both offering very different feature sets): Geopher Lite ($2.49AU the first geocaching application) - this was the first geocaching application released for the iPhone, it allows you to search for nearby caches based on your current gps co-ordinates.
http://geopherlite.blogspot.com/ iGeocacher ($17.99AU the one I ended up buying) - allows you to upload gpx files to perform paperless caching, does not seem to support dynamic (online) searches for nearby caches from geocaching.com - this means you need to set up your quick queries and perform a data load in advance, however it also means that you do not need 3G or wireless Internet access when you are out on the road.
http://www.ayefon.com/geo/index.cfm Geofinder ($1.19AU) - simply lists the nearby caches in safari.
http://www.whatsoniphone.com/node/2551
Looks like geocaching.com are ramping up to release an official geocaching app for the iPhone (at a price of $9.95US). However, it will not support the load of gpx files (which for me is the most important feature of a geocaching application).
http://forums.groundspeak.com/GC/index.php?s=465d64e0a32c3583d6d1674a5cb54a22&showtopic;=204713&st;=0&p;=3654012&#entry3654012 I'm sure that we will see more over the next few months!