I saw Marie's comment about Breaking Dawn, so I decided I'd do some research. I didn't find any summaries, but I did find this piece of info on Stephenie's website [stepheniemeyer.com].
The countdown to Breaking Dawn will begin on Saturday, May 31, 2008 with the publication of the Eclipse Special Edition which coincides with the annual BookExpo America show, the book publishing industry's largest convention. This deluxe hardcover version of the third book in the series not only includes the cover art for Breaking Dawn, but also the first chapter—both of which will be revealed to the public for the first time. The trade paperback of New Moon, the second book in the Twilight Saga, will also be released on Saturday, May 31, 2008.
So... it looks like the cover art AND first chapter will be in the special edition of Eclipse. I'll keep an eye out for summaries, though. [: