http://kachibito.net/wordpress/youtube-tips.html
<?php if(get_post_meta($post->ID, 'YTid', true)): ?> <iframe width="649" height="395" src="http://www.youtube.com/embed/<?php echo get_post_meta($post->ID, 'YTid', true); ?>?rel=0" frameborder="0" allowfullscreen></iframe> <?php endif; ?>