Valve entering the field with a sort of convenient-lanbox-console would be a MAJOR game changer, and a HUGE threat to both MS and Sony since they'd probably be competing fiercely for the same demographics. Of course, if a steambox and rumors of Sony's PS4 being virtually a pc dovetail... then that'd be even more amazing, and a pretty awesome partnership by Sony I'd think.
Nintendo, of course, would also be severely challenged, but insulated a little bit more by their exclusive first-party development and wider consumer range.
Since a proposed steambox probably wouldn't support motion control or a Tablet Controller, it's stuff like this that shows just how important exclusive hardware/interface features, which some call "gimmicks", can be.