"The Dark Below" controversy continues as time passes, TMView shows that the application for the trademark is under examination so the name remains as being registered by Blizzard so we might actually see an expansion after this.
What we know from Wowpedia and Wowwiki
"The Dark Below is a place inhabited by demons, devils and infernal creatures, from where some sorcerers take their power directly and sometimes have a patron beast to foster their magic."
Relationship to the Twisting Nether
The demons from the Dark Below apparently exist apart from the Burning Legion, who reside in the Twisting Nether. The Dark Below may be another description for the "Twisting Nether", in which case these are demons of a separate faction that live within the same plane of existence. This would explain how player warlocks are able to summon demons that do not serve the Legion.
The "Below" in The Dark Below
This implies a region that is underneath the crust of Azeroth, and not the twisting Nether in outerspace up in the sky. In early conceptions, hell is percieved to be underneath and not up in the sky.
Edited: Link to the trademark: http://www.scribd.com/doc/159754556/...80%AD%E2%80%AE
Second update:
It looks like Blizzard registered another trademark, for Diablo 3 this time, "Reaper of Souls"
Link: battle.net/d3/en/reaper-of-souls/
Update: Finally we have the trailer for Diablo 3 Reaper of Souls expansion live , wierd that they wanted to unveil now instead at Blizzcon.