The Steambox was hypothesised and many pieces do fit with Valve's current hardware push (and hiring of electrical engineers) however Valve said they're not close to doing anything like that. The new hardware guys seem to be focusing on input devices too, such as the controller with swappable elements. Jeri Ellsworth is an interesting new employee, you should go do some research about her. Lastly the rumour about Tim Cook meeting with Valve and potential Apple / Valve hardware development isn't going to happen because Tim Cook never met with Valve.
So back at square one. The closest thing to the steambox is the new Steam big picture mode (10' UI), which can be used on other screens in the home, such as a big TV. Presumably this is going to apply to realms where Steam is likely to be on a TV set, possibly for PS3 users since SteamWorks already exists there. The 360 is a less willing partner, I doubt MS will want it. Some PC / Mac gamers use TV sets as an extra display to play games so perhaps the UI will also be aimed at these individuals too. I don't know how big that market is but it may be big and the Steam hardware survey says so!