XHTML 1.0 served as text/html

Forgive the multiplicity of named recipients, but
I am very uncertain as to whom to address this :

There has been a fairly protracted discussion recently
concerning the pros and cons of serving XHTML documents
as text/html  or as application/xhtml+xml, but I was more
than a little surprised today to discover that when the
W3C (HTML) validator is asked to validate

	http://www.isg.rhul.ac.uk/

it states that the (page) is "Valid XHTML 1.0 Transitional"
without issuing even a warning that it is being served as
text/html rather than application/xhtml+xml.  Now it is
clear from Section 5.1 of

	http://www.w3.org/TR/xhtml1/

that this is acceptable, yet

	http://www.w3.org/TR/xhtml-media-types/

also states clearly that

	"application/xhtml+xml SHOULD be used for XHTML Family documents"

My question is therefore : should not the validator issue
a warning when this last guideline is ignored ?

Philip Taylor