IMO, the largest problem with the idea of HTML converting to XHTML is 
attempting to change the MIME Type, but not changing the extension.   
Though that is only a solution for simple setup; there are much more 
complicated problems with the conversion... the largest one being the 
current market leader doesn't support true XHTML, throwing the whole 
option out.  And unfortunately, not all user agents correctly send back 
what file types they can handle, so checking the HTTP headers on the 
server side is only a solution for certain browsers.

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