WordPress: "Add HTML5 Shim To Your Theme’s Header" - Add this function to your theme’s functions.php file.

gistfile1.php

// add HTML5Shim to your theme's header
function html5_shim () {
    echo '<!--[if lt IE 9]><script src="http://html5shim.googlecode.com/svn/trunk/html5.js"></script><![endif]-->';
}
add_action('wp_head', 'html5_shim');