Old versions of IE that do not understand the new tags in HTML5 require a little JavaScript trick, called a 'shiv', to enable them for display and styling. Setting the CSS to display: block
alone will not do the trick.
The HTML5Shiv can be included by itself, or packaged with a wider library such as Modernizr (which you really should be using in any case).
HTMLShiv: http://code.google.com/p/html5shiv/
Modernizr: http://modernizr.com/