Just read something important from this "Basic Security in iOS 5" article on Ray Wenderlich's site that:
never save sensitive information to NSUserDefaults – these values are stored in PLAIN TEXT (i.e. just like this blog post) for all to see (and is also saved inside of your application bundle). Be mindful of what you put in there.
Which is quite interesting as I never know it's stored in plain text and also saved inside application bundle.