Here is what we are using at http://www.develop.com/webcasts to keep people from accidentally leaving the page while a video is playing or paused.
$(document).ready(function() {
var video = $("video#webcast_video");
if (video.length <= 0) {
return;
}
window.onbeforeunload = function () {
var htmlVideo = video[0];
if (htmlVideo.currentTime < 0.01 || htmlVideo.ended) {
return null;
}
return "Leaving this page will stop your video.";
};
}