Forbes has gotten word from Valve's Doug Lombardi that AMD will be involved in Steam Box. Since you can roll your own Steam Box that was not in question. What is in question is how will AMD's drivers stack up.
“Last week, we posted some technical specs of our first wave of Steam Machine prototypes. Although the graphics hardware that we’ve selected for the first wave of prototypes is a variety of Nvidia NVDA -1.56% cards, that is not an indication that Steam Machines are Nvidia-only. In 2014, there will be Steam Machines commercially available with graphics hardware made by AMD, Nvidia, and Intel. Valve has worked closely together with all three of these companies on optimizing their hardware for SteamOS, and will continue to do so into the foreseeable future.”