Lots has been suggested about doc types needing to be spot on for
SEO to work properly - but I've read (almost certainly here) about properly executed experiments by SEOers who've come to the conclusion that, from an
SEO point of view, the doc type doesn't matter a jot ... Furthermore, in case you didn't know, neither Google nor Yahoo have a doc type declaration. Says it all for me. If you don't believe it, visit them and view source.