As previously anticipated, Chrome OS is a Linux distribution that includes a custom version of Chrome as the default browser. Google has recently posted a build of the custom Chrome and some people installed the .deb package (it's no longer available officially, but you can download it from other sources).
Chrome for Chrome OS is not very different from the browser you can already install in Windows, Linux or Mac. The main difference is that the browser includes UI elements from a traditional operating system: clock, battery status, network settings.
Since Google Chrome will include most of the features that are necessary to use an operating system, it's obvious that the browser is the only visible component of Chrome OS, a lightweight browser-centric operating system.
There's no reason to anxiously wait for Chrome OS, when you can already use Chrome in your favorite operating system and get the same features. A metaphor has become reality, but the result fails to impress.