UUID

//ios6

SString *UUID = [[NSUUID UUID] UUIDString];

//ios5

- (BOOL)application:(UIApplication *)application
didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    NSString *UUID = [[NSUserDefaults standardUserDefaults] objectForKey:kApplicationUUIDKey];
    if (!UUID) {
        CFUUIDRef uuid = CFUUIDCreate(NULL);
        UUID = (__bridge_transfer NSString *)CFUUIDCreateString(NULL, uuid);
        CFRelease(uuid);

        [[NSUserDefaults standardUserDefaults] setObject:UUID forKey:kApplicationUUIDKey];
        [[NSUserDefaults standardUserDefaults] synchronize];
    }
}