<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional with HTML5 microdata//EN" "xhtml1-transitional-with-html5-microdata.dtd">
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict with HTML5 microdata//EN" "xhtml1-strict-with-html5-microdata.dtd">
... or more likely, override the DTD used for validation by your particular XML validating setup.
BTW, here's something interesting, and I only include this as it's useful to know when answering the question. I honest to God thought that using the above doctypes would invoke quirks mode when rendering. Turns out, much to my great surprise, that IE8, Chrome 14, Firefox 5 and Opera 11.50 all render such a doctype in Standards mode. Who would have thought! So you could, if you wanted to, upload your XHTML pages onto the public internet with the custom doctype and the newer browsers at least would do the right thing.
I had need of extending XHTML 1.x Strict with schema.org/HTML5 microdata and getting it all to validate properly for updating nedprod, and Microsoft Expression Web has the occasional tendency to eat bits of HTML it edits, so validation is handy for catching when it borks. Hence I have created these DTDs which extend the standard XHTML 1.0 ones:
Since the major search engines have decided on schema.org last June (2011) as the way to do rich snippets, this question has become much more important seeing XHTML5 does not yet have a working DTD (BTW, http://www.html5dtd.org/ is working on a XHTML5 DTD and may well be ready when you read this, if so disregard what I'm about to say). And what I am about to say summarises a page I placed at http://www.nedprod.com/programs/portable/XHTMLwithHTML5microdata/ a few weeks ago, and there has more detail including a rich snippets demo if you want it.
To use, take a copy of your desired DTD (don't use the original from nedprod, I can't afford the bandwidth) and use as follows: