On Steam, the games are labelled as "supporting Linux", by which they usually mean some version of Ubuntu, which Steam runs on now. Other distributions often work too, usually with some small tweaks that can be googled.

If "SteamOS" becomes a target in the Steam app, this general "Linux" label may be pushed to the sidelines, and since SteamOS may contain more proprietary parts than general Ubuntu does, we may find it much more difficult to run the games on any distribution other than SteamOS itself. That was my point.

Steam seems to install Linux games in such a way that it'll run on any Linux distro. I know Steam works on Arch-based systems, openSUSE, Ubuntu-based systems, and likely more...

Also, from the sound of the announcement, Valve plans on keeping the OS bits open, so I see no reason they'd break compatibility with other Linux versions (and it wouldn't be in their interest since Linux is becoming more and more significant).

Agreed. I would imagine they will continue trying to support major linux distros such as Ubuntu, but it's not plausible for them to make sure it work on other Linux distro. Interested 3rd parties though will probably try to keep most things ported to other Linux distros though.