The following infos are coming from an interview with tigole during the Games Convention:
- Sunwell Plateau will be released before the expansion "Wrath of the Lich King", Tigole said its planned for a patch after Zul'Aman (so it might be 2.4 minimum to get it). Sunwell Plateau is a new instance which can be found in Quel'Thalas (forest region north of the continent of Lordaeron). The instance is divided in 2 parts, one of them is designed for a 25-man raid instance (said to be the most diffcult in WoW, harder than Black temple), the other part is a 5-man instance.
- Tigole talked about a new instance "Nexus" located in Borean Tundra, it should be the first instance you will zone in when entering Northrend by Borean Tundra; this instance will be divided into several wings and the hardest wing is designed for a 25-man raid. Nexus (thats the name of this new wotlk instance) is located at the top of a mountain, and you will encounter Malygyos in it, the blue dragon.
A video of this interview should be up this week end.
Source