Blizzard has said its upcoming games will be here at Gamescom and playable, but they've been a bit coy about which games specifically that means.
You'll be happy to know that we've seen World of Warcraft, StarCraft II and Diablo III all on the yet-to-be open show floor. While I can't confirm that Diablo III will be playable, it certainly looks like it will. The game has a sizable booth, but it was also closed off—likely because of the graphic, demonic nature of the Diablo universe—which lends a bit more credence to the idea that the game will be playable.