After ages and ages, the XHTML2 Working Group of the W3C has published an updated to their core document - XHTML Modularization. This update is currently a Proposed Edited Recommendation. Assuming all goes well, it will become a formal recommendation in about a month. You can see more about it, review the changes, and comment on it via
http://www.w3.org/TR/xhtml-modularization.