Are you after tutorials on specific lighting, shading and texture techniques in Photoshop? If so, there's quite a few articles online, if you know where to look.
Apple's own iOS HIG is an essential read.
Smashing Magazine has lots of tutorials and info about designing for iPhone/iOS:
In terms of overall design patterns and UX, I recommend taking a look at this article: iPhone application design patterns.
If you want to learn a little about the code, this may be worth a look (the first tutorial is free): Design Then Code. A bit of Xcode and Objective-C knowledge can be a great thing for designers.
Sebastiaan de With has created a great iOS icon template that a lot of designers use.
There may be some good tutorials on PSD Tuts, although I haven't read many of them (I've been using PS for over 20 years, so I... umm... don't really need most tutorials).
Like Davide Di Cillo, I highly recommend Tapworthy as well. Josh knows what he's talking about. I think even experienced designers would get a lot out of the book.
I recommend installing Developer Picker, which lets you copy colours in a format that are native to Cocoa. Very handy when designing iOS apps.
I've also written some articles on the topic. They're usually more technical than creative — they're not about achieving specific UI looks, but more about setting your Photoshop documents up to be scalable, remain editable and be easier to export.
Here's some articles I've written that may be of interest:
Oh, and definitely make sure you set up Photoshop so colours don't get altered when exporting: Setting up Photoshop for web and iOS development. Absolutely essential.