The announcement was made by BigWorld Technologies, not Sierra, and it didn't reveal much about the nature of the game, except that it it will be (according to Gamasutra) "targeting a mass-market audience." King's Quest Online? Probably not, but if only! If you want to know more about BigWorld, Ten Ton Hammer did a great interview with one of the guys behind it earlier this year.
Oh, and just so you know; Sierra is/was owned by Vivendi Games, and so is (like Blizzard) now part of the epic corporate monstrosity known as Activision Blizzard. All your studio are belong to us.