Valve never sold out because they became the "out" other companies sell out to. They successfully built a revenue-capturing money-printer in the form of the Steam store and service and now they don't have to make games at all to keep their bottom line strong. Not to imply they shouldn't have; get that gold ring and all.

(But I may also argue the point they never sold out in terms of being a game studio as opposed to a publisher.... "So when's Half Life 3 releasing?")