Secondly, Yogg-Saron's Place was supposed to be below Vordrassil (As a Sidenote, there is a Dead end in Vordrassil that looks like some kind of Cave entrance), that part however was a remainder of the Plan that Yogg was supposed to be in Azjol-Nerub, not Ulduar, however it was a major point of Grizzly Hills lore, remaking and explaining that giant broken tree would have been harder to explain.
and a statement from BlizzCon 07:
"One of our jobs as adventurers is to explore Ajol;Nerub to see what is there. We will find something bigger and ancient and... ok it's obviously one of the old gods. Ok, we've already let that out of the bag. (laughter)"