I see on many many sites, people are serving their documents as 
text/html but using the xhtml 1.1 doctype.  Yet they don't think 
anything about it since the validator says it's valid.  Even trying to 
contact them letting them know usuaully results in a response like "if 
the validator says it's ok, then I'm not changing it"

I think you should add a small modification to the markup validator so 
it will call any document with a doctype of xhtml 1.1 and served as 
text/html invalid.