Yep. It's gotta be Yogg-Saron in Ulduar... or atleast affecting Ulduar.

I have the distinct feeling that the first few bosses in that place will be Tyr, Hodir, Freya and Mimir due to some mind controllyness that forces them to do the Old gods will. Swiftly followed by Thorim quite likely and then maybe Loken(If his demise in HoS was soemwhat exagerated) before possibly Yoggy himself. Makes a bit of sense though I was also under the impression that Yoggs was under Grizzly Hills trying to grow a tree. So possibly we'll see a Yogg related lackey if Loken doesn't fit that role himself.

I only hope we can liberate the watchers rather than slay them in the battles. I've really enjoyed the lore of stormpeaks so far and it'd be a shame to slay all of those guys... i'd rather like to have a quest chain to return them all to their temples again(phased perhaps)

Another interesting thing to consider is the Faceless ones under Icecrown, how closely related is Mr Saron to the Lich King? First name terms? I've got to say future content patches have me most intrigued in Wrath.