title: is there a notification when "slide to unlock" has occurred
body: <p>I have an iphone game that plays background music using AVSoundPlayer - when someone locks the iphone the music stops which is fine.  But when someone unlocks it, I don't want my music to start playing again while you're staring at the "slide to unlock" screen - I want it to start playing once you've actually slid the button and the app is visible again - is there some way to detect this?  (I've tried  applicationDidBecomeActive but that fires when the phone is unlocked but not when your app is visible yet...)</p>