Originally Posted by kovidgoyal
@HarryT: The metadata cover in a MOBI file is not part of the normal book contents, so you cannot point to it in the OPF guide.
You are right of course. I read ePub and Mobi books with about equal frequency and was forgetting which opens at the cover (ePub) and which does not (Mobi). In the absence of a "firstpage" Guide item, however, a Mobi book will open at the first page of the book, and (in all Mobi implementions that I'm aware of) a "page back" from there will display the cover.
As a matter of good practice, however, I think personally that it's always as well to explicitly specify in the Guide where the book should open.