In his farewell message on Gamespy he says he'll be creating content for Blizzard games and World of Warcraft specifically (so be on the lookout, as he writes, for "quest instructions (shouted) at you in a very loud, very poor Scottish accent"). Kosak has been responsible for hearthstones being memorably described as "the chickenrock," an explanation of aggro mechanics, The Five Stages of Warcraft, and the ultimate goblin-engineered weapon (which you can find in Area 52).
Unfortunately, it looks like he's not going to be able to continue the "Flintlocke" comics while at Blizzard (whether for time-related or legal reasons, I'm not sure), so that's kind of a downer. The current Horde comic has already been planned all the way to its end, which will occur at some point in late April. I was disappointed to read this -- I love Flintlocke, and the Horde storyline is amazing ("What kind of combat skills you got on that thing?" "Probably flee and mate") -- but I expect we'll be seeing some of Kosak's influence ingame.
Congratulations, Mr. Kosak, and from all of us here at WoW Insider, good luck!