Chromium OS is the open source version and is available for download in source form right now. You'll have to compile it yourself, I think. I'm pretty sure things will stay that way.

Chrome OS is the closed-source version that is sent to vendors to be installed on computers. I'm pretty sure Google won't make Chrome OS available for download, since there will be no single "Chrome OS" image; it'll be customized for each hardware option.

(I think.)