I just found this discussion about XHTML on the site of Anne van Kesteren. In the last comment Faruk Ates hopes that the Dutch government will make more valid websites in the future.

Well, he doesn't have to wait any longer. In the last few months I've had the privilege at Q42 to build the new Dutch Government's Advisory website. As this advisory is telling all Dutch Government sites that they should make valid websites, we had to make this site as valid as possible. The CMS generates XHTML 1.1 with mime-type application/xhtml+xml for browsers and other clients that support it, and HTML 4.01 for browsers that don't. The markup is meaningful and so are the URLs.

In this case it was not that hard. If your CMS is based on XSL, te output is automatically well formed XML. With well formed XML creating valid XHTML is just as hard as creating valid HTML. And a simple last transformation can convert the XHTML to HTML. To convert other sites like the main government site is going to be a lot harder. But if it wasn't hard, it wouldn't be necessary to convert it.