DarkNecridObviously, but the topic is about the SteamOS itself and the original post you are all quoting/replying to was talking about the SteamOS itself being open source, which it will be.
Not necessarily. An operating system is more than just a kernel, so it depends on how much Valve changes (and whether they open source those changes), as well as where you draw the distinction between operating system and user applications.
To elaborate some more on what I mean... Do you consider the init system a part of the operating system? Basic tools such as coreutils and the shell? What about the display server?
I would argue that the existence of Steam and other Valve tools are integral in making SteamOS what it is and should therefore be coupled with the notion of 'SteamOS'. So what if some of the foundation applications are open source, they already were, so nothing changed.
Then again, even if you don't agree with my definition that the user interface should be considered a part of 'SteamOS', it still doesn't guarantee that the operating system in its entirety will be open source. It's entirely possible Valve could replace parts of the traditional stack and keep them closed source. For example, they could write their own display server. They could even take existing open-source code and not release their modifications, as not all of the standard Linux libraries and applications are released under the GPL, but under more lenient licences such as BSD or MIT.