All the major browsers have been supporting the HTML5 doctype for a year now I would say its time to drop the XHTML doctype. But even on that note it looks like about 50% of users have browsers that are not compatible with HTML5 (IE8, IE7, Firfoxe 3.6, ect.), grr. People should have a year to upgrade after that they may find we don't write code for their browser. I would guess once Windows 8 hits later this year we will see much of that get resolved, but I don't see much reason not to start migrating to the HTML5 doctype now.
With the advent of HTML5, I've been wondering: is XHTML still relevant?