Not exactly a bug, making some notes about minor issues I've just been looking into, and maybe it can be improved? Entering something in the Meta Content Profile in Site Settings (like "http://dublincore.org/documents/dcq-html/") causes a site to fail the W3C validator, saying "The profile attribute on the head element is obsolete. To declare which meta terms are used in the document, instead register the names as meta extensions. To trigger specific UA behaviors, use a link element instead.".  This is with an HTML5 skin. I think the value should instead be written out as

<link rel="profile" href="http://dublincore.org/documents/dcq-html/" />

(noting that this particular URL is flagged as superseded).

Also when entering DC on a page (Page Settings), we get "The scheme attribute on the meta element is obsolete. Use only one scheme per field, or make the scheme declaration part of the value."  So on the Meta Element, we need to leave the Scheme field empty in order to validate successfully. So perhaps that property could be suppressed when the doctype is html5, or dropped altogether?

As to what to put in the rel property of the Meta Link, this seems to be a bit of a minefield of incomplete specifications. Most sources (e.g. http://wiki.whatwg.org/wiki/MetaExtensions) suggest including this:

<link rel="schema.DCTERMS" href="http://purl.org/dc/terms/">

(which can be done in mojo at page level using the Page Settings > Meta Link, or included in layout.master) however the W3C validator still chokes on this, claiming "Bad value schema.DCTERMS for attribute rel on element link: The string schema.dcterms is not a registered keyword" (which appears to be wrong or daft).

Whether any of this makes much difference in the real world is another matter, but there may be a couple of changes to make from the above?