Here it is from Blizzard; the official news on Silithus changes in the upcoming 1.8 patch:

Since the beginning of recorded history, Azeroth has seen the most terrible conflicts imaginable. The War of the Ancients and the Sundering of the World. The coming of the Tides of Darkness. The return of the Burning Legion, the Scourge of Lordaeron and the rise of the Lich King. Yet all this time, deep beneath the shifting sands of Silithus, an unspeakable horror has been lying in wait, ever patient, and ever enduring...

To learn more about the changes coming to Silithus, take a moment to read the Introduction to Silithus...

http://www.worldofwarcraft.com/info/underdev/1.8/silithus-intro.html

...and check out the new Silithus map.

http://www.worldofwarcraft.com/info/underdev/1.8/silithus-map.html

Silithus seems to be making a huge change, and it looks good!