This scrap of code is a WordPress Plugin. it allows me to write something like [youtube id="p5ACl4iBGd0"]

The code then outputs either the XHTML 1.0 embeding code or the XHML-MP code which links to the mobile version of the site

function youtube_func($attr)
 {
 //Get the name of the Wordpress Template being used
 $name = get_template();

 //If this *isn't* a mobile theme
 if(stristr($name, 'mobile') === FALSE)
 {
 //Link to embeded YouTube video source
 $ytURL = "http://www.youtube.com/v/" . $attr['id'];

 //XHTML 1.0 Strict way of embeding movies
 $s = "<object type="application/x-shockwave-flash" style="width:450px; height:366px;" data="" . $ytURL . "">"
 . "<param name="movie" value="". $ytURL . "" />"
 . "</object>";

 return $s;

 }
 else //This is a mobile theme
 {
 //Link to mobile YouTube
 $mYtURL = "http://m.youtube.com/watch?v=" . $attr['id'];
 //Image of the video
 $mYtIMG = "http://i.ytimg.com/vi/" . $attr['id'] . "/default.jpg?w=128&amp;h=96";

 $s = "<a href="" . $mYtURL . "">"
 . "<img src="" . $mYtIMG . "" />"
 . "</a>";

 return $s;
 }
 }
 add_shortcode('youtube', 'youtube_func');

Many thanks to the Wordpress API, this excellent Wordpress Plugin Tutorial, and this Valid XHTML YouTube embed code generator.

You can download the plugin from WordPress.