title: NSUserDefaults, do something on first load only
body: <p>I'm trying to do show an alert the first time the app is launched.  I'd also like to show this alert if they go into a settings page later on and want to see it again.  I figured I could set a boolean in the <code>NSUserDefaults</code>.  I'm not quite sure how to do that though (assuming this approach is right).  I thought in <code>applicationDidFinishLaunching:options</code>, I could get the value for key <code>@"FirstTimeLaunched"</code>, and in <code>applicationWillTerminate</code>, I could set the object for that key to <code>NO</code>.  If this is the case, how do I get the <code>BOOL</code> to be <code>YES</code> for the first time?  </p>
  
  <p>Also, do I need to set it to <code>NO</code> in <code>didEnterBackground</code> mode as well?  Thanks.</p>