Yes, THIEF use the same game engine as System Shock, but I question this because I saw this game on steam and it's available to steamOS, or there's something behind that I don't see?
And THIEF runs almost perfectly, almost...(little issues that make the experience less enjoyable, but not so painfull).
I think, Valve has their own windows compatibility layer or maybe it's wine. I know they do have their own software which can translate directx9 to opengl.
I guess a few games have been released with it.