Building for 10.5 in Xcode 3.2 (on Snow Leopard) error
NSApplicationDelegate is a new protocol as of 10.6. You're getting the error (I'm guessing) because your application delegate is implementing this protocol. I'm not sure if this is the best practice on this, but you might just consider using the preprocessor to help you:
#if (MAC_OS_X_VERSION_MAX_ALLOWED <= MAC_OS_X_VERSION_10_5)@interface MyAppDelegate : NSObject#else@interface MyAppDelegate : NSObject <NSApplicationDelegate>#endif