From threeA Wiki
Valve Corporation, also known as Valve Software or Valve, is an American video game development and digital distribution company made famous for its multiple critically acclaimed video game series Half-Life, Portal, Left 4 Dead and Team Fortress, and its social-distribution networking software Steam.
The quote at the bottom of the advert, "Ordinarily, I wouldn't contemplate them... But these *are* extraordinary times," suggests that the collaboration will be based around, or at least launched via the Valve intellectual property Half-Life. Ashley Wood clarifies on 3A Forums: "refers [to] all valve, ALL. and shit its gonna be epic" 
The first item that has been revealed is a Weighted Companion Square² which, in a humorous sense, poses as a teaser of the collaboration between ThreeA and Valve.