Well, that didn't last long. PC Gamer reported back in December 2012 that Valve had taken on a couple of San Francisco-based employees, formerly of Star Filled Studios. According to Valve-eyeballing fan site ValveTime, though, that small team has now departed after less than a year with the venerable developer/publisher, going their own way with a new studio called Temple Gates Games.
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.