If your XHTML page is served from the server with a mime type of text/html, then then you're not really serving XHTML, you're serving HTML. If your server is configured to serve the page with a mime type of application/xml+xhtml, then browsers (that support it) will treat it as XHTML.
You should read the thread Why most of us should NOT use XHTML [webmasterworld.com]. It's almost 7 years old, but it's still referenced often. :)