If you wanted to add the cover page to the HTML TOC that is generated (or getting it to show up in the NCX version of the TOC), it would probably be better to add the cover manually using Tools - Table of Contents - Edit Table of Contents:
http://web.sigil.googlecode.com/git/..._contents.html
You can do something similar to also add the copyright page, and any other Front Matter of the book as well (Dedication, Frontispiece, etc. etc.), without having to clutter them with <h1>, <h2>, <h3>, ...