Difference between Google Chrome OS and Chromium OS

Google Chrome OS is to Chromium OS what Google Chrome browser is to Chromium.
Chromium OS is the open source project, used primarily by developers, with code that is available for anyone to checkout, modify and build their own version with. Meanwhile, Google Chrome OS is the Google product that OEMs will ship on Netbooks next year.
Therefore Chromium OS is the source code released by Google which can be used by any developers to build and create their own OS. Chrome OS is a end product that will be released by Google and developed from this Chromium OS as the base.