Since no one commented on my previous post about the dangers of MIME types, (if any,) I went ahead and did it anyway. You can now add ?mimetype=type to view that page served with a different mimetype. So now, if you are running Mozilla Firefox 1.5 Beta 1, you can go to: http://www.kronkltd.net/wp/?p=720&mimetype=application/xhtml%2Bxml and see what I was trying to do.
Note: The character ‘+’ gets translated to a space in url’s, so in order to specify a mime-type such as application/xhtml+xml, you have to replace the ‘+’ with a ‘%2B’.