Okay - this took way too long. But finally the XHTML Media Types document has been updated! This document, first released in 2002, was meant to illustrate the various media types that XHTML could be served as, and how to decide when to use which. Unfortunately, the document was not really maintained and quickly became irrelevant. This new version is much more focused than the previous one. It also gathers together in one place this guidance and the information about what to do and NOT do when writing XHTML that can be delivered to HTML user agents (like Internet Explorer) and also to XHTML user agents (like Firefox). This is a MAJOR step forward.
Still more to do, of course. The world of standards never sleeps. It does, however, rest its eyes from time to time.