I too want Wherigo for the iphone. Someone earlier in the thread mentioned the iphone sdk wouldn't allow it. NOT TRUE. I've had several apps built for my web stores that interact with store databases via web based api calls. It completley legal for apps to do this. It's also completely legal for apps to interact via wi-fi or bluetooth with other devices. The Wherigo iphone app could be designed to allow use of a bluetooth gps device. This would satisfy even the most difficult situations where prime accuracy was of great concern. The gps built into the iphone doesn't have to be the only that the app can use. Also it's possible to just build the app as a shell that pulls in code from the Wherigo website. all that is needed is an api of some sort to request data in an iphone gui format. So long as data access is available, the app could go directly to Wherigo.com to get info.
ideally it would allow use of 'downloaded' cartridges when not in data areas too.
I do envision though, that the Wherigo app would be used mostly by the less serious and more famly fun oriented types of cachers. If someone were to want to get to remote areas and the most difficult of caches, they'd probably be buying better more specific equipment for that kind of sporting anyway.
The Wherigo app on the iphone would attract new users to the fun of geocaching.