The application delegate will call the method upon successful registration of remote notification after you call this method in your UIApplication:
(void)registerForRemoteNotificationTypes:(UIRemoteNotificationType)types
According to: http://developer.apple.com/library/ios/#DOCUMENTATION/UIKit/Reference/UIApplication_Class/Reference/Reference.html
When you send this message, the device initiates the registration process with Apple Push Service. If it succeeds, the application delegate receives a device token in the application:didRegisterForRemoteNotificationsWithDeviceToken: method; if registration doesn’t succeed, the delegate is informed via the application:didFailToRegisterForRemoteNotificationsWithError: method. If the application delegate receives a device token, it should connect with its provider and pass it the token.
Now, to elaborate further, normally an app will call the registerForRemoteNotificationTypes in your didFinishLaunchingWithOptions:(NSDictionary *)launchOptions in your application delegate. And therefore, the application:didRegisterForRemoteNotificationsWithDeviceToken is then usually called moments after the launch of the application.
Edit: The application:didRegisterForRemoteNotificationsWithDeviceToken still gets called for subsequents registration after the first.