HTML5 markup won’t be enabled by default, because that could break child themes released before it was added to Genesis 2.0. Therefore, even after Genesis 2.0 is released in final form, HTML5 support will have to be enabled on a site-by-site basis.
To accomplish that, we’ve released a small plugin, wpPERFORM.com Genesis Enable HTML5 Support, that’s available on all plans that does one simple thing: it adds theme support for HTML5 markup for theme’s running under Genesis. With the plugin activated, your site will output HTML5 markup in place of the old XHTML markup.
With HTML5 enabled, your site may require some CSS changes. Enabling HTML5 support via a plugin let’s you use the plugin as an on/off switch to examine the impact on your site. If the changes are minor and easily implemented via custom CSS, you can stick with the new HTML5 markup. If they’re more drastic, you can develop the custom CSS over time and only leave HTML5 markup enabled when you’re satisfied with the final result.