Valve has a tremendous amount of positive mindshare among PC gamers, and they've built that up over the course of years. I expect they recognize that strong-arming customers into doing or buying something many of them may not want is the most efficient way to destroy that mindshare.
Based on the wording of last week's announcements, I don't expect them to be that foolish. HL3 only needs to be *compatible* with Linux to spur support for SteamOS, it doesn't need to be a SteamOS exclusive of any kind. Valve seems to be focused on keeping the PC as a platform; SteamOS intends to simply make extending that platform to the living room easy enough to step on the toes of the big console makers.
Biting the hand that feeds IT
