Yes! A new Valve game. And while we currently have no idea what that game might be, we also kinda do. We know that Valve hired Defense of the Ancients Allstars developer IceFrog, which almost certainly means they're developing something in a top down, small-unit-tactics multiplayer vein. Oh, and Valve also filed the trademark for DotA, and there was a tweet from a voice actor saying he had fun at Valve "recording for DotA", but that's since been removed.
In any case, according to both IceFrog himself and GameInformer, GameInformer will be posting all the details of this mystery project later today. Presumably much later today, when the Americans have woken, bemused and horny, from their enormous beds. Stay tuned.