I wrote this about the Mac, but it applies to the iPhone and iPad as well:
1) Mac users are highly sensitive to the quality of your products' user experience. What this means is, go native or don't bother. Even though Google Earth and Photoshop are rife with UI atrocities, don't imagine that you can get away with ignoring the rules like they can. They're 500-pound Gorillas, and you're not. If you are Google or Adobe, get with the program and write a Cocoa UI, already. It's about time.
2) The native language for the Mac and the iPhone is Objective-C. Get used to it; it's not hard to learn. Any developer familiar with C should be able to learn Objective-C in a day, and be an Objective-C language lawyer within a week if he cares to. Yes, there are Ruby, Python, and other bridges you can use, and they work just fine, but limit this to integrating existing libraries with your apps. DO NOT try to use the bridges as a way to avoid learning the environment you're working with.
3) A cross-platform GUI is neither feasible nor desirable. You can't #ifdef the difference between Cocoa, xlib, and Win32. Don't believe me? Look at OpenOffice. (If OpenOffice looks OK to you, then please, forget about offering your products on the Mac. You'll only cause us pain.)
4) Don't bother with third-party cross-platform GUI libraries like Qt. Yeah, you can make it sort of work, but you'll get a lot of complaints from your Mac customers, and it will be more expensive than properly factoring your code and writing a native GUI for each platform. For every Mac customer who complains about a bad UI, there are many more who took one look at it and decided never to do business with the vendor in question.