Valve (the people who made Portal, Half-Life, Team Fortress 2) are making their own OS. They’re also going to ship two of their own consoles: a $100 box that streams games from your computer, and a $300 one that’s standalone.
Steam is already extremely popular, and has a TV-centric mode, so if they can take that mode and get rid of the whole needing other people’s OS to run Steam games thing, this seems like it could really work. I’m not sure it changes Nintendo’s situation that much (which is not great), since their success is based more on creating fun games with unique gameplay, but if I were Microsoft or Sony, I’d be shitting myself.
The Verge has a big article on the whole thing.