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