Might dump it onto a separate SSD and have a play, wonder if it's a openSUSE or Debian fork. Probably Debian since a lot of their early work was on Ubuntu.
It's a custom distribution not based on Debian or any other distribution.
As promised, the OS is built on Linux (not based on Ubuntu, we're told, but entirely custom), though you'd never know it as the only interactive layer is all Steam.
Oh I wasn't suggesting they haven't put significant work into tailoring Linux for their own needs, simply citing their early test bed. Ubuntu itself uses Debian package manager and the Steam client has been served up in .deb form on Linux until now, so it's likely that will be the package manager bundled up with the rest of the OS.
Well I guess we'll find out either way in 2 days. Woo!