Just FYI… WordPress 3.2 includes an updated TinyMCE which allows HTML5 tags. It’s still under development but is fairly stable if you want to take the risk of running it now.
The alternative is to write any posts requiring the tag with the visual editor turned off. As long as you don’t turn the editor on again on that same post the tags will stay there. Just turn the editor on once you have navigated away from that post in admin.