Blizzard releases official Gnomeregan and Echo Isles Cataclysm information

• Operation Gnomeregan: Transcript and details of the event
• Operation Echo Isles: Transcript and details of the event

Of course, it's important to remember that these transcripts were taken from the PTR and could easily be changed in upcoming patches. Nothing is set in stone until it happens.

The full Blizzard announcement is as follows: 

Nethaera -- An Assault on Two Fronts

High Tinker Mekkatorque, betrayed by Mekgineer Thermaplugg and forced to flee the irradiated city of Gnomeregan with the few of his fellow gnomes who managed to survive, is preparing to reclaim his once glorious city. Meanwhile, in distant Durotar, Vol'jin of the Darkspear trolls is determined to exact revenge upon the witch doctor Zalazane for dividing the Darkspears and seizing the land bestowed to them by Thrall: the Echo Isles. Read more about the ongoing struggles of the gnomes and the trolls... and their plans to seek redemption.

Gnomes: http://www.worldofwarcraft.com/info/underdev/3p9/gnomeregan.xml
Trolls: http://www.worldofwarcraft.com/info/underdev/3p9/echoisles.xml
 

Regarding "Patch 3.9": The URLs above point to these events happening in patch 3.9. It is possible there will be a patch 3.9, however it is also possible the "9" simply stands for "the very last patch before 4.0" (patch 4.0 is the Cataclysm content patch). No one outside of Blizzard knows the right answer, and don't read too much into URLs.

Source: wow.com