The Lenny xserver does not work with the balloon framebuffer by default due to this bug: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=521060
Here is a workaround until the above bug is fixed.
Make a file containing this info:
xserver-xorg xserver-xorg/config/device/use_fbdev boolean true xserver-xorg xserver-xorg/config/device/driver select fbdev xserver-xorg xserver-xorg/config/device/driver string fbdev
Then set the config like this:
debconf-set-selections debconf-test dexconf