While it would be nice to have a toggle that enables output of HTML5's new semantic tags, I don't believe that it should be the default. Internet Explorer 6-8 require a JavaScript polyfill to support these tags, which means that for IE users with JS disabled, the layout completely breaks. Here's a blog post from Trevor Davis at Viget that summarizes the argument pretty well: http://www.viget.com/inspire/html5-elements-irresponsible-ch...
