I don't plan on using the iPhone GPS for geocaching, I really don't think it'll give me the type of features or accuracy that I would need for a serious day in the woods, but I do think there are ways the iPhone would be useful while geocaching.
One is the ability to know about nearby geocaches. Sometimes you don't really need accuracy, especially in urban settings, and if you're out and about and notice that there could be a quick geocache nearby, the iPhone would be helpful. I used to bring my GPS with me to unfamiliar cities because it does contain information on nearby businesses, restaurants, and shops. This feature is more useful on the iPhone as the Garmin does not constantly update (and I paid a lot of money for new updates a few weeks ago). The Garmin is much more useful for travel though, and I have a car mount for it so I don't see the iPhone replacing that either. And as far as I can tell, I'm not able to load my waypoints into my iPhone anyway so that won't be useful for geocaching at all. The setting and capacity I'm in will really determine if I need to have the Garmin or if I can be ok with just the iPhone, and for serious geocaching I plan on using both.
I did use my old phone for looking up caches, and I think paperless geocaching is where the iPhone can really be useful. Unfortunately there are no good apps out for this right now. I did find a few for the old iPhone, which I really wasn't happy with. Yes, you can visit the geocaching.com site on your phone, but the site is bad enough on a regular browser when it comes to crashes and poor interface design. I would use the WAP site on my old phone (one of the very few sites I could access on the internet, and the reason I added the option). This will still work on the iPhone, but its features are limited, and at least on my old phone I had to log in each time I wanted to look up a cache, a slow process that I would usually try to avoid (the phone only really came out when I absolutely needed it). I will probably still bookmark this site as it will likely come in handy.
Logs are not always necessary for geocaching, and I've hacked up my waypoints using GSAK (Geocaching Swiss Army Knife) to make them even less necessary. GSAK is an awesome and super powerful database for managing geocaches (it's not flashy, but this thing rocks). I run all my pocket queries through there to reformat the waypoints on my Garmin to include cache type, difficulty, cache size, and even if the last log was a find or not. These are displayed in either the waypoint name or notes area on my GPS. It was a GSAK feature I randomly stumbled across, but it's really come in handy for not having to rely on paper logs or the WAP site all the time.
GSAK also has useful features for offline viewing which have helped in areas where I couldn't get a cell signal (and the rare occasion I also have my computer, usually caches along a route to a destination). Since the logs are stored local on your computer until you re-update the database you can access them without internet. It also allows you to export html files of the logs, which is what I decided to do to view caches on my iPhone. The html files are then uploaded to heidigoseek.com where I can access them on the phone. The formatting and features of these exported html files are much better than the WAP site. Although they are not real time updates, links to the caches are provided. I get a pocket query in my email every Friday so I'm ready for geocaching on the weekend. The file is usually only 1 or 2 days old.
I believe there may also be a way to store the html files on your iPhone for offline access. This would only be useful if geocaching in an area with no signal. This has only happened in one trip while caching in Northern Wisconsin and the UP (not a big surprise), and I haven't had much of a problem accessing the online site in any other area so I'm ok with online upload. I normally don't cache in an area without checking on access capabilities though.
I think with a combination of the Garmin and the online capabilities of the iPhone, geocaching will be a lot more organized, and easier. But I'm still holding out for a useful iPhone app.
