Google presented work in progress on Chrome OS yesterday and further details have been published on the User Experience page of The Chromium Projects website.
Image 1: How the whole screen may look
Chrome OS will look very familiar to those how use Chrome browser.

Image 2: The tabs
The tabs at the top will feature bigger icons to better distinguish between open websites and applications.

Image 3: Applications
Clicking on the Chrome OS icon, on the top left, will open a window featuring shortcuts to websites and online programs. Think of this as Google Chrome OS’ version of Window’s start menu.

Image 4: Notifications
Chat and notification windows pop up from the bottom of the screen.

(Images copied from The Chromium Projects website and used under the creative commons licence)
To see Google Chrome OS in action view the video showed at the presentation of the project.