New changes on iOS 5 has cause application using the default MainWindow.xib will break on the new SDK. 

Make sure you add the RootViewController

RootViewController *rootViewCont = navigationController.visibleViewController;

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions { }

All iOS 5 applications should have a root view controller

Found a great sample of how make modifications to your existing application or to start a new app here:

 

#import <UIKit/UIKit.h>
@class RootViewController;
@interface AppDelegate : UIResponder <UIApplicationDelegate>

@property (nonatomic, retain) UIWindow *window;
@property (nonatomic, retain) RootViewController *rootViewController;
@end 


and
 


#import "AppDelegate.h"
#import "RootViewController.h"

@implementation AppDelegate

@synthesize window = _window;
@synthesize rootViewController = _rootViewController;

- (void)dealloc {
	[_window release];
	[_rootViewController release];
    [super dealloc];
}

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    self.window = [[[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]] autorelease];
    // Override point for customization after application launch.
    self.window.backgroundColor = [UIColor whiteColor];
	_rootViewController = [[RootViewController alloc] init];
	self.window.rootViewController = self.rootViewController;
    [self.window makeKeyAndVisible];
    return YES;
}

@end