Wouldn't it be a laugh if Valve made Half-Life 3 a Linux exclusive? I haven't played any of the Half-Life games (HL2 gave me motion sickness) but I know that they are a cultural touchstone for many a PC gamer. Valve aren't economically dependent on making games, so they could do it if they want. Gabe Newell has been on record saying that Win8 is going to be bad for PC games (Though I am not sure if I agree with him on that one). So it's certainly not that a crazy thought

In the end, I imagine that they probably won't make it Linux exculsive because even Valve aren't that crazy, but it would certainly make things very interesting for PC gaming