Most of the ePubs I have seen include a single page titlepage.xhtml or title_page.html (or whatever) as the first item in the spine. Some just include one large xhtml file, and then whatever is on the 1st page is the "cover" page as far as Adobe Digital Editions is concerned. I have also seen some cover pages listed in the spine with the linear="no" attribute, but that may be dangerous if there is ever an ePub reader that treats such items differently to linear="yes" (see
OPF 2.0: Spine).
An ePub can have a "guide" section in its .opf file, but it is optional and Adobe DE ignores it. This means that OEB-based ePubs, which are entirely legal ePubs, won't necessarily show their cover image when viewed by Adobe DE. It is now common for MOBI ebooks to only have the cover in a guide item of type "other.ms-coverimage-standard", so these won't automatically convert to Adobe DE compliant ePubs without special handling.
There are advantages to the "first page is the cover page" approach though. For one thing, it does not have to be (or include) a bitmapped image.