CVG have noticed that the official Gamescom website lists Valve in the strategy section. This very strongly suggests that we'll see DOTA2 in Cologne next month as the Dota follow-up is the only strategy game Valve are known to be working on at the moment. We anticipated this when we first heard Valve would be attending the show, but it's still very exciting. We'll get to see whether the recently leaked DOTA 2 patch notes are correct.
Of course it's possible that Valve aren't showing off DOTA 2, but instead revealing a mystery strategy game, but that seems unlikely. Whatever they're showing though it's definitely not going to be Half Life: Episode 3, not that that ever stops the perpetual rumours.