I'm going to take a stab here.  I believe it will be a contemplation of *all* the known D&D gods/demigods.  It would only make since, being a D&D product.  I believe it will consist of the gods from Greyhawk, OA, FR, etc...  Kinda like a more in-depth look into the nature of gods, how they are related and what-not.  Probably going to be alotta relation in the book to the planes themselves.  Probably descriptive upon exactly *what* a diety is, *how* they obtain their power, and *where* exactly they are.  Might be some info as well on how a person can create their own diety (yea, it's possible and still maintain the D&D perspective).  You can surly expect more than just FR and Greyhawk.  In my opinion, it won't really be worth getting, it sounds like, for the most part, just being a more in-depth, organized set of things you should already know.  Anyways, they may throw in some diety-related skills, traits, spells, etc.  Should be interesting to see what exactly is put in there, but knowing WotC, it should be pretty obvious.