Alongside shipments of the first beta Steam Machines to users, Valve has released a FAQ with info on how SteamOS functions for users and developers. While it's pointless to recopy the entire FAQ in a news post when you can go and read it now, here are a few interesting and relevant facts for the game development community:

SteamOS is open source

"All of the base operating system components are open source. The Steam client itself is proprietary, as are some proprietary third party drivers."

Though it's built on Debian Linux, you'll be OK developing on Ubuntu, Valve's recommended desktop distribution

"All Steam applications execute using the Steam Runtime which is a fixed binary-compatibility layer for Linux applications. This enables any application to run on any Linux distribution that supports the Steam Runtime without recompiling. As long as your development environment targets Ubuntu 12.04 LTS with the Steam Runtime, it will run without change on SteamOS. If you are a Steam partner, see the Linux development page for more details on creating Steam applications for Linux and SteamOS."

Hardware support should "largely" mirror vanilla Debian

"The drivers for SteamOS are provided as part of the system image and integrated by Valve. Valve will be integrating new and updated drivers over time. The process for installing new drivers should not be different from any other distribution in that the Debian community or Valve will re-package new driver releases and re-distribute them. If an end-user wants to install a new driver package on top of their existing SteamOS installation we expect to remain largely compatible with the Debian packaging scheme."

You can easily get access to a regular Linux desktop on SteamOS, even though it boots directly into Steam

"SteamOS is designed to run Steam and Steam games. It also provides a desktop mode which can run regular Linux applications... To access the SteamOS desktop, it must be enabled from the Steam Settings menu." For the rest of the details, read Valve's full FAQ.