Tigole from Blizzard has posted recently about some of the upcoming Instance/Raid content for WoW. Here is the statement:



Blackwing Lair is going to be capped at 40 people. The zone will be tuned to be extremely difficult to present a challenge to those guilds who have experience with content such as the Molten Core and Onyxia.

But rest assured, we have been listening closely to the community's desires for a more "casuaul" friendly raid zone. So the zone we're working on after Blackwing Lair will be tuned for a raid somewhere around 15-25 people (most likely 20). Ultimately, playtesting will determine the final cap. Also, we want the zone the be doable much more frequently than our "hardcore" raid content. So most likely it will have a reset of 1-3 days. We're also trying to make the zone require less time than Molten Core. This zone is called Zul'Gurub and is located in North Eastern Stranglethorn. As it gets closer to the time of release, we'll be releasing more information. But I think this zone will be a very attractive stepping stone for smaller guilds into the higher level raid content.