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.