Phoronix showed back in December that Intel HD Graphics work with SteamOS and AMD Radeon GPUs with Catalyst also worked. However, Valve didn't officially advertise the support and encouraged the use of NVIDIA GeForce graphics over outstanding bugs.
Yesterday I mentioned that AMD provided a preview Catalyst Linux driver to Valve that was incorporated into SteamOS. This new AMD Catalyst Linux driver reportedly works much better for many Steam Linux games. With this new driver, plus several Intel HD Graphics Linux issues having been sorted out separately, they have went ahead and declared Intel + AMD graphics officially supported alongside NVIDIA graphics.
Right now the AMD Catalyst driver is used by default for Radeon GPUs but the open-source R600/RadeonSI Gallium3D drivers for AMD GPUs should also be possible given that SteamOS is carrying the Linux 3.11 kernel and Mesa 10 for Intel open-source support.
Other details of the latest SteamOS changes as of 8 Janaury can be found from this SteamCommunity.com page.