With XHTML, mime type really makes a difference, especially 1.1. At least in my experience it does. I'm finding that pages will render completely different if the mime type is text/html or application/xhtml+xml.
And from what I can tell, application/xhtml+xml is the future! Yay.
In PHP, this can be done like this:
ini_set('default_mimetype','application/xhtml+xml');
I was trying to figure this out to use the nice shjs package, but for some reason it wouldn't work without the right mime-type.
Oh, but of course Microsoft Internet Explorer doesn't like this. Thankfully there is a rough fix, but I'll probably just have use a separate stylesheet to render the output as html instead of xml. Good ol' MSIE.