(Note: I don't know how long this has been the case. I think it's recent)Previously, the page for the main menu would be inaccessible to anyone without an @google.com email address (read: Google employees only). That page now redirects to http://welcome-cros.appspot.com/menu, and it only needs any Google account. If you use the Developer Tools ... thing, you can make a sidebar visible.
What's interesting is that the file browser entry links to http://kan.homedns.org/fb/, which I think was used in the original presentation of Chrome OS. Instead of having some cool integration with Microsoft Office files, it directly linked to Microsoft's Live editor. (Pretty weak, I know.) Obviously that feature's still in development.