Third party apps likely have a way to load/save documents, or that places a BIG yet-unstated limit on the kind of app you can me. You therefore need a way to browse and open said documents. Sounds like a "Finder" to me! Obviously simpler than the Mac Finder.
The Finder could be just a kind of file open/save dialog, but if it's accessible on its own, without launching any other app first, then it's more Mac-like. (The difference being that if you don't launch an app first, then you can browse ALL filetypes, opening whatever app is needed for the file you choose.)
I hope the iPhone will be able to browse (view icons/names) for files that it CANNOT open--just to review what's stored--and I hope it can browse files you've simple dragged onto the phone from a Mac/PC like it was any other HD. (The way you can drag files to an iPod, if you set that option.)