Blizzard has reportedly trademarked the name 'Cataclysm', with fans now running rampant with speculation that this is the third expansion to World of Warcraft.
The name certainly fits into WoW's lore and so seems a highly plausible rumour, meaning team Blizz are working on the South Seas area of Azeroth. BlizzCon '09 may reveal more...
Frankly it could be any number of things, like a fancy new system or thingy for Battle.net, or even the name of an unannounced title in the works outside of Warcraft. We'll just have to wait and see what this August holds when BlizzCon kicks off.