application/xhtml+xml and forms

This e-mail regards my problems with my forms and sending the containing
XHTML1.1 document of those forms with the correct mime-type in
application/xhtml+xml

Now I've read Ian's XHTML advocacy document a few times, though I did not
see anything regarding forms.

Let's get to the point..
I am using forms for a contact script [ http://tim.textdriven.com/contact/ ]
On all the pages of my site I've used PHP to send the correct header to
browsers that understand it. Firefox gets the document as
application/xhtml+xml handling the document somewhat correctly. The problem
is with browsers that handle it correctly (Firefox) your input is forgotten
when you hit back when redirected to the error page.

For example:
Type anything in the Message * field then submit the form omitting the other
required fields. You are taken to an error page, when you hit your browsers
back button to fill in the missing fields your message is then mysteriously
gone.

This is only when the document is sent correctly as application/xhtml+xml
This does not happen with IE, probably because the document really delivered
as text/html

I know this form problem is due to sending the document as
application/xhtml+xml because I removed the PHP script that sends the header
and my message input was remembered when I hit back.

What am I missing here with sending the document correctly?
There must be some specification I can read up on about this.

Thank you for your time,
Kind Regards

--
Tim Callahan
http://tim.textdriven.com