I spotted an early release copy at a local Border's today and paged through it. Being ambivalent about 4E, I don't have a solid opinion about it, but one thing I can report on:
The Blood War still exists--it's just in a state of uneasy truce right now, since Asmodeus doesn't want to provoke the Abyss, which would only unify in the face of an outside threat.
We also find out why it started in 4E's cosmology--the Abyss wants a piece of its Heart back. Asmodeus took it long ago, forged it into his famous ruby rod, and used it to kill his master. This weakened the Abyss and demonkind, too.