Well I'd like to say thank you to you guys for an informative thread.
I started coding (what I thought was) XHTML a few years back because I was under the illusion that it was how I should do it. I've never done anything with the MIME types though. So, even though the pages validated AND worked in IE, it turns out it was never actually XHTML.
Now I know what I know from reading this thread I'm going back to HTML4.