In the original announcement, the phrase "a new windowing system" lead many bloggers and commentors to believe that Chrome OS will not have the X server, which is used in essentially all Linux desktops. However, three additions to this list has me believe that Google meant a new window manager, (e.g. Metacity or Compiz) which would certainly be much less drastic. xserver-xorg-core and xserver-xorg-video-intel (link) along with xscreensaver are included.
Speculation: the window manager will be a fork of Metacity developed by Google. It will support compositing (it is used in many places in Chrome), though it will be trimmed down from normal Metacity.