Use you a CFUUID! No ARC for you!

gistfile1.txt

    NSString *uuid = [[NSUserDefaults standardUserDefaults] stringForKey:@"CFUUID"];
    if (!uuid) {
        CFUUIDRef cfuuid = CFUUIDCreate(NULL);
        uuid = [(NSString *)CFUUIDCreateString(NULL, cfuuid) autorelease];
        
        [[NSUserDefaults standardUserDefaults] setValue:uuid forKey:@"CFUUID"];
        [[NSUserDefaults standardUserDefaults] synchronize];
        
        CFRelease(cfuuid);
    }
    NSLog(@"UUID: %@", uuid);