In order to get IE to work properly with HTML5, you have to initialize the new HTML5 elements before the page loads.
The HTML5 Shiv script will do that for you. Include the following conditional comment in your head.
<!--[if IE]>
<script src="http://html5shiv.googlecode.com/svn/trunk/html5.js"></script>
<![endif]-->
Or you can download it directly from http://html5shiv.googlecode.com/svn/trunk/html5.js. Not suprisingly, this script is thanks to Rem.