Oops, sorry, I was looking at the HTML of the theme previewer instead of the theme itself. With regard to WordPress, only my "XHTML friends network" point remains valid. Concerning web development in general, I have seen way too much cargo cult boilerplate, too many hardcoded lang attributes, etc.
Yeah, the value of "XFN" is debatable. But it's usually written as a link element that doesn't really break anything.