universal main.m for iOS and OSX with or without ARC. It checks the info.plist and invokes apple's native `UIApplicationMain` or `NSApplicationMain` with the right parameters in any case. (See Comment for more info)

main.mm

//
//  main.m
//
//  Created by Dominik Pich on 10.02.13.
//  Everybody is free to use this however they want!
//
#if TARGET_OS_IPHONE
#import <UIKit/UIKit.h>
#else
#import <Cocoa/Cocoa.h>
#endif

#define kDDAppDelegateClassKey @"DDAppDelegateClass"
#define kNSMainNibFileKey @"NSMainNibFile"
#define kNSPrincipalClassKey @"NSPrincipalClass"

#if TARGET_OS_IPHONE
#define kNSPrincipalClassDefault @"UIApplication"
#else
#define kNSPrincipalClassDefault @"NSApplication"
#endif

id DDAppDelegate = nil;//arc releases this too early if we make it a stack var

int main(int argc, char *argv[])
{
    int status = -1;
    
#if !__has_feature(objc_arc)
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
#else
    @autoreleasepool
#endif
    {
        NSDictionary *infoDict = [[NSBundle mainBundle] infoDictionary];
        NSString *appDelegateClassName = [infoDict objectForKey:kDDAppDelegateClassKey];
        NSString *nibName = [infoDict objectForKey:kNSMainNibFileKey];
        NSString *principalClassName = [infoDict objectForKey:kNSPrincipalClassKey];
        
        BOOL hasAppDelegateClassName = appDelegateClassName.length;
        BOOL hasNibName = nibName.length;
        BOOL hasCustomPrincipalClassName = (principalClassName.length && ![principalClassName isEqualToString:kNSPrincipalClassDefault]);
        
        assert((hasAppDelegateClassName || hasNibName || hasCustomPrincipalClassName) && "need to specify EITHER a xib file (via NSMainNibFile key) OR a class name to use as delegate for the application (via DDAppDelegateClass key) OR the name of the custom principal class (via NSPrincipalClass key)");
        
#if TARGET_OS_IPHONE
        status = UIApplicationMain(argc, argv, principalClassName, appDelegateClassName);
#else
        id appDelegate = nil;
        if(hasAppDelegateClassName) {
            Class appDelegateClass = NSClassFromString(appDelegateClassName);
            DDAppDelegate = [[appDelegateClass alloc] init];
            [[NSApplication sharedApplication] setDelegate:DDAppDelegate];
        }
        status = NSApplicationMain(argc, (const char **)argv);
#if !__has_feature(objc_arc)
        [appDelegate autorelease];
#endif
        appDelegate = nil;
#endif
    }
#if !__has_feature(objc_arc)
    [pool drain];
#endif
    
    return status;
}

one main to rule them all ;)

a better, unified main.m for OSX and iOS

A main.m file is the entry point for all objective-C mac and iOS apps. In most cases it is a simple 'one liner' that start a NSApplication or UIApplication which then configures itself from the info.plist.

But that simple approach has one crucial drawback:

It demands a xib file that contains the appDelegate -- and in many cases only that! Since the iPhone 5 not even the main window is created in the xib due to the different screen sizes

one more issue you don't have to worry about

Writing a custom main.m file is far from rocket science but it gets annoying soon and is just one more piece of custom code!

universal main.m implementation

This main file checks the info.plist for a xibfile (NSMainNibFile), a specific XXApplication principal class (NSPrincipalClass) and a class to use as appdelegate (DDAppDelegateClass)

It then invokes apple's native UIApplicationMain or NSApplicationMain with the right parameters

fixed an arc issue

Why you declare id appDelegate = nil? In which case it used?
I think DDAppDelegate should be used instead appDelegate.