Well, obviously all PCs are Steamboxes, technically speaking. However, if it's a streamlined box that plugs right into a television and boots right to steam, I could maybe see them putting up a stink, even if the branding was sufficiently ambiguous.
The MistBox ;)
I don't know, though. To be honest, I'm kind of shocked no one has tried this yet. It seems like such an obvious winner, even if there is a slight chance you would go to court with Valve.
I was thinking the other way around - that they might be perfectly encouraging of selling such a device, just that using 'Steam' (their TM) in the name or advertising without permission might get you in trouble.
Otherwise I can't see an impediment. Maybe Valve could even have a 'designed for Steam' certification program or something.