gistfile1.js

// player is the MTVNPlayer.Player object created by the embed api
player.on("stateChange:paused", function(event){
  // we don't want to do this for flash
  if(MTVNPlayer.isHTML5Player){
    if(!event.target.element.webkitDisplayingFullscreen){
      alert("show your poster screen, we're not in full screen");
    }
  }
});