Wow, I'm making a fool of myself today. I've actually yet to have big and small work (though looking back, that may have more to do with the context in which I've tried using them, on LJ and DW, where they don't seem to work, at least for me, since I don't normally use them in my standard development, in favor of CSS). It seems <i>, <b>, etc will be deprecated in XHTML 2.0, though (though that's a ways off still).