graeme_p - 12:20 pm on Jul 2, 2011 (gmt 0)

Dive Into HTML5 has great examples of things that will improve a site if you use HTML 5 AND which will either degrade gracefully on older browsers or which can be detected and replaced javascript to provide the same features.

I can see no reason NOT to use HTML5 on a new site or a new design on and old site. Even if you have CMS that generates snippters of XHTML or HTML 4 it will almost all remain compatible.