Chrome OS will also be open source. The main area of the operating system is Chrome itself. Google includes the source code for the Chrome OS version of Chromium/Chrome in Chromium's version control. Changing some build system files and compiling the modified version allow you to have a version of Chromium, with Google's design for netbooks. Chromium OS is the unofficial version of Chrome OS.
I have compiled Chromium OS. (As far as I know, I was one of the first outside of Google to do so.) Chromium OS is under active, open source development. Being the Chrome fan that I am, I set up this blog to track Chromium OS development and news. I'll update my copy of Chromium OS and run it. I'll see what changes there are and take screenshots, and I'll post them on this blog. And very importantly, I'll share my copies of Chromium OS, so you can play with them too. I'll also share the changes I made to make Chromium OS compile, so A) you can hack on it too, and B) Google doesn't sue me for violating the license. :)
Final tally: 12(+1) Chromia, 9(+1) Chromes