I just finished creating the ePub for my own novel yesterday, and it doesn't look like you can specify an opening page. However, you can tell it what order to open the files, so you can set it to open at the cover file, or at the table of contents file, or whichever other file you wish.
With mine, I have individual xhtml files for the cover, copyright page, each chapter, and the various end matter sections. So in my content.opf file, in the <spine> section, I arranged the files in the order I want them to open. This way, I know it will open on the cover, then move on from there to copyright, TOC, etc.
If you wanted to start it within one of the files, though, I'm not sure you could. But then I'm new at ePub, so don't quote me on that!