yeah, cuz I would assume that the PC version and the Steam box version of the games would not be compatible with the opposite platform. So I dont know that steam would allow to play PC library on the SteamBox...but I could be wrong...they could just enable the other platform download for free if you own it on the other already. That would be sick
You can already play cross platform between Windows, Linux, Mac, and Android ( when available ). Steambox will be running Linux.