1) XHTML 1.1 is NOT real world deployable -- you have as little business using it as you do HTML 5 -- which is to say NONE. In fact, I would say browsers support for XHTML 1.1 is inferior to that of HTML5.
You want XHTML on a website you should be using XHTML 1.0 STRICT... which frankly is the most recent W3C recommendation I would even THINK about using on building a page.
Remember, XHTML 1.1 doesn't even HAVE a backwards compatibility section, which is a hefty part of why not one real website has adopted it for use... and the handful of niche users who have tried generally are in the "we don't care if it doesn't work" crowd.
2) You should probably be including language encodings -- not just the lang= and xml:lang= but also the language meta.
3) given the relatively simple layout I'm wondering what 90% or so of your DIV are even doing in there.
4) This is unrelated to your issue, but it might be a good idea to smack turdpress upside the head to force out all the bloated garbage they saddle you with for no good reason -- like the classes on EVERYTHING and the bandwidth wasting full URL's on EVERY link.
There was a decent article on doing that recently here on sitepoint.
Which of course wouldn't be neccessary if the people writing wordpress had the faintest clue how to write HTML/CSS properly.
I also suspect that some of your php is "suspect"/sloppy given the use of single quotes in the output. Probably not causing any issues, but it's an indication of bad practice (and needlessly complex php coding).