You can of course use services like vekja.net or mybitcoin.com on a mobile browser, depositing money there to the extent you trust them.
I think that's the best option right now. Like cash, you don't keep your entire net worth in your pocket, just walking around money for incidental expenses.They could make a smaller version of the site optimized for mobile. If there was an app, it could be a front end to one of those, with the main feature being QR-code reader, or maybe there's already a universal QR-code reading app that web sites can be designed to accept scans from.
If there was an iPhone app that was just a front end for vekja or mybitcoin, not a big involved P2P, would apple approve it and if not, on what basis? It could always be an Android app instead. An app is not really necessary though, just a mobile sized website.
A web interface to your own Bitcoin server at home wouldn't be a solution for everyone. Most users don't have a static IP, and it's too much trouble to set up port forwarding.