Hi Malcolm,
The new doctype is legal, and is the one recommended by the html 5 spec (and is backwards compatible). I'd be interested to know what the problem is you are getting. Ultimately it will be better to fix the problem, than fix the doctype in the long run.
However it is returned by the w3header method in the WebHandler - so you can add code there to return whatever you like as the doctype (do the return before the parent call.)
cheers
Bruce