A few days ago Blizzard registered the trademark for Warlords of Draenor in New Zealand and Australia. Tonight, the same trademark has appeared in the US and Europe. The current speculation is that this might be the next World of Warcraft expansion; however, there has been no official announcement from Blizzard yet.
We'll all know the truth about WoW's next expansion this Friday at BlizzCon 2013.
UPDATE: Post edited to reflect that Warlords of Draenor also just appeared on the US trademark database tonight.
To see these results for yourself, you can go search on this page. Search the term: "tm:WARLORDS OF DRAENOR"