I have an iOS 4.2 universal app. I've tested the app against all available simulators (iPhone 4.0, 4.2 and 4.2; iPad 3.2 and 4.2). Everything runs great on these simulators and on real devices with iOS 4.2 (iPad 4.2 and iPhone 4.0).
The problem I am running into is while testing the application (from Xcode) on a device with pre 4.0 iOS---iPhone 3.x. The error is that it fails to run the application with:
dyld: Symbol not found: _OBJC_CLASS_$_UIPopoverController
The application builds with no erros (base SDK is still 4.2), installs okay but crashes while being launched. I have been careful in placing all iPhone only code in /iPhone folder/group, iPad only in /iPad folder/group and all shared libraries and views in /Shared. Furthermore, the application runs with no error on my iPhone with iOS 4.0. The UIPopoverController is never used/called from the iPhone nib so I am at a lose as to what is causing this. I am looking into adding a new target that doesn't include any of the post iOS 3.2 additions. Any help on this in the meantime would be greatly appreciated.
Switch your compiler to LLVM GCC 4.2 and follow all the other steps in this blog post by Marco Arment: Supporting older versions of iOS while using new APIs.