Chromium/Chrome OS is an OS developed by Google that uses the Linux Kernel and the Google Chrome Browser as its User Space. I ran it back when all those Beta Netbooks were being shipped and as a good generalization "Chrome OS is a system for those who have a computer to surf the web and nothing more". Pretty much just a computer with a web browser and nothing else.
Not sure if anyone is looking at porting it. Since all the distros have the ability to install Chromium, you can get the same affect by running Chromium full screen and logging into Google. The major downside to running COS is you can't install software that is not directly loaded in Chromium. All of the "apps" are web/cloud applications. Want to edit a word document, Chromium sends you to Google Docs.
Google might have some special Chromium-only plugins or add-ons but highly unlikely.
More info on Chrome OS
On my netbook and Pi I run Arch Linux. At work I use Debian for my development platform and have my own internal built distro for products.