Here it is in a nutshell:
- Browsers decide how to handle a file based on the MIME type that the server sends with it.
- HTML Web pages are identified with a MIME type of text/html.
- Pages written in XHTML that are sent with a MIME type of text/html don't benefit from any of the features of XHTML.
- To benefit from the features of XHTML, pages must be sent as application/xhtml+xml.
- The most popular Web browser (Internet Explorer 6) cannot view pages sent as application/xhtml+xml. </UL>
And all this time, I was ignorantly assuming that any well formed HTML page was XHTML. Well, you live and you learn.
UPDATE: I forgot to link to Ian Hickson's paper http://www.hixie.ch/advocacy/xhtml.