There seems to be a problem with the X Server drivers etc in Ubuntu 9.04 64bit (Alpha-6) Jaunty when Guest Additions is installed.
The Guest Additions installer complains about an unknown X Server version and refuses to install the software that manages window resizes etc.
Also at re-boot the OS complains that there is a XServer already running on display : 0. A few dialogs appear and finally the OS starts the Xserver on display : 1.
Display size is only 800 * 600 and no Guest Addition screen size features are enabled.