OK so I understand that HTML5 supports the existing markup (HTML + XHTML). This is great. However, what happens when I want to use any of the new HTML5 tags?

If I want my website to work properly in IE6 should I avoid using any of the new HTML5 tags?
(Just for the record, which browsers currently support HTML5 and CSS3, and which ones do not? Are there tried and true fallback methods for those older browsers?)