Valve will be attending this year's Gamescom, according to Go Nintendo.
It actually seems they've got something to show. The only confirmed project Valve is working on is DOTA (Defense of the Ancients) 2 - a fantasy strategy game for PC and Mac. But Valve could, of course, announce further DLC for Portal 2.
Perhaps a glimpse of Episode Three? (ahahahahahahaha)
Gamescom will run from August 17 to 21 in Cologne, Germany - make sure you don't miss anything!