This should be little-to-no surprise for the viewers of our podcast, as this story was discussed there, but Valve has confirmed AMD and Intel graphics are compatible with Steam Machines. Doug Lombardi of Valve commented by email to, apparently, multiple sources including Forbes and Maximum PC.
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 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.
Ryan and the rest of the podcast crew found the whole situation, "Odd". They could not understand why AMD referred the press to Doug Lombardi rather than circulate a canned statement from him. It was also weird why NVIDIA had an exclusive on the beta program with AMD being commercially available in 2014.
As I have said in the initial post: for what seems to be deliberate non-committal to a specific hardware spec, why limit to a single graphics provider?