Valve brought Steam (in the form of a subset of the Steamworks APIs) to PS3 for cooperative play in Portal 2. But they can't expand Steam to someone else's platform, those three you mentioned already have their respective stores.

As I see it, a "Steam Box" would merely be a hardware specification that Valve sets and game developers can rely on, ship with Steam preinstalled and supporting their previously announced "big picture mode", and the multi-input controller. But for all intents and purposes, it would be a standard desktop PC. Probably one designed to be used in the living room, but standard hardware nonetheless.