This plugin allows you to change the DIV/Container element of each widget to one of the new HTML5 Semantic tags.
Yes and no. This plugin will work modifying the containers, but themes like 'TwentyTen' use unordered lists
ul to surround the widget areas and this will fail w3 validation since
<ul> cannont contain elements other than
You can edit the sidebar.php file in the theme editor and remove
<ul class="xoxo"> and
</ul> for both the primary and secondary widget areas. Leave the
<div>s and you should now validate.