For the record Kevin: both Flightcrew and Epubcheck claim that the attribute "opf:scheme" is not allowed in the creator element. Same for the contributor element.
This is what the editor produces now:
Code:
<dc:creator opf:file-as="Hope, Anthony" opf:role="aut" opf:scheme="marc:relators">Anthony Hope</dc:creator>
Older versions of the Metadata Editor produced the same thing--but without the opf:scheme="marc:relators" attribute/value.