The Hive interviews Gabe Newell talking to Valve's Managing Director about Half-Life and its follow-ups. Here is a quote on the subject of the newly announced official add-on from GearBox:

Newell: We knew the guys at Gearbox. Doug Wood here at Valve was actually one of the founders of Rebel Boat Rockers. Charlie Brown and Gary McTaggert had been doing some consulting with RBR on Prax Wars shortly before coming to Valve. So we had a lot of confidence in their team, and we were excited by the ideas they came up with for creating a complementary experience for Half-Life.