I know the current fad is to serve web pages using XHTML doctype, but this is, in almost all situations, wrong. OK, count to ten, and read on.
Unless you are going to serve your pages with a mime-type of application/xml (which virtually nobody does), and therefore make your pages innaccessible to the 65% of your viewers who use Microsoft browsers - you should NOT use the XHTML doctype.
I do sympathise with the WordPress developers, who've clearly gone to a lot of effort to provide output that validates using the W3C HTML validator. However, whilst admirable it is, nevertheless, a mistake, and if WordPress is really interested in standards and valid markup they should head back to HTML - awaiting the release of HTML 5 (the likely successor to HTML 4.01).
This is a rather technical issue and to explain it fully needs more detail than I can afford here. However, there are some clear explanations of the major issues here:
and a more technical discussion here:
Please either revert back to HTML as the default, provide the option to select it in the Dashboard, or write a plugin that will translate back to HTML (thereby allowing WordPress to serve valid markup).