Hey, welcome to geocaching! I fell in love with it after finding my first cache and haven't looked back since. I'm a gadget geek at heart, so having a mobile device and a GPSr just seemed like the natural activity to me :-)
Just want to let you know that there is already an application in the works for the iPhone platform that does just what your looking for called Geode. I'm currently working on this project as lead UI (and part time Public Relations guy), and as a dedicated geocacher, I can tell you that this is one fun app to use. You can load geocaches on to it, browse from a list and choose one to navigate to, have an interactive map, and a compass that will allow you to change function (north up, track up, or track up with cache pointer). I know I sound a *little* biased as I'm a part of the project, but I wanted to start this project because I was tired of lugging around my GPSr and having to use my iPhone to browse GC's website when I got stuck or needed some more info. This application started off as a list of "You know, if I could create an app" much like what you posted. We're very much listening to the community on features to add, so your input would be VERY much welcomed!
Take a look at our site and tell me if this was something you were looking for (at least for the iPhone side of things)
Geode
BTW - I know this is my first post, I've been having crazy account issues with this forum. Bleh :-P