You need to include the HTML5 shiv script
in order to allow styling of HTML5 elements in older IE browsers: http://code.google.com/p/html5shiv/
To use, include the following script in your element above your CSS:
<!--[if lt IE 9]>
<script src="http://html5shiv.googlecode.com/svn/trunk/html5.js"></script>
<![endif]-->
See -> html5 new elements (header, nav, footer, ..) not working in IE
Another option is to use Modernizr, which includes the HTML5 Shiv and also provides HTML5 feature detection.