Add IE conditional html5shiv to header - http://css-tricks.com/snippets/wordpress/html5-shim-in-functions-php/
<?php
// add ie conditional html5 shiv to header
function add_ie_html5_shiv ()
{
echo '<!--[if lt IE 9]>';
echo '<script src="http://html5shiv.googlecode.com/svn/trunk/html5.js"></script>';
echo '<![endif]-->';
}
add_action('wp_head', 'add_ie_html5_shiv');
?>