IE8, XHTML and xmldap.org

Some time ago I changed the HTML code that the xmldap.org site produces to XHTML.
It seems that IE8 is not happy with it, although I tested all pages with http://validator.w3.org/
Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.1; Trident/4.0; .NET CLR 2.0.50727; InfoPath.1; .NET CLR 3.0.04506.648; .NET CLR 3.5.21022; .NET CLR 3.0.4506.2152; .NET CLR 3.5.30729)
Sad. When I use IE8 and Cardspace to present an Information Card then IE8 offers to store a file to my local disk... When I post that file's content to the validator it verifies that this is valid XHTML 1.0 strict. And the content-type is "application/xhtml+xml". Maybe this is the problem?

Don't know whether I should care... Google does not consider IE8 to be a suitable browser (taken from here). Firefox is my browser and I assume that the others implement xhtml correctly too.
Anyways, if one IE-enthusiast offers a solution that is standard conform then I am happy to improve the xmldap site.