I have Graal Online Classic on my iPod, have been playing it for months now. So far that might be the closest thing to a Zelda game on the iphone even though it is unofficial, it borrows everything from ALTTP from graphics to gameplay and items.
It should be easy to pick out the similarities here.
But as far as an actual Zelda app goes, I actually hope it would be no more complex than ALTTP. Controls on an iphone touchscreen are not great and I doubt I would do well in a iphone version of PH or ST. A simpler game such as LoZ would be hard enough to play without any more complex controls or items beyond move and attack with sword. Maybe if Nintendo actually went out of their way to produce simple touchscreen games meant for the app store and not just weird hard to play ports such as the Sonic the Hedgehog series. But I do not see that happening since Nintendo already has their own handheld device with it's own online downloable content. So anything showing up on an iphone would be just as strange a concept as a Nintendo game appearing on a Sony device.
That does not mean some crafty individuals cannot make an "unofficial" port of some kind into an app. People would have to grab it fast before it disappears though.