I asked myself this very question today and I think I have it solved, at least for now. Bumblebeed is an upstart service, so when looking to edit the startup script for it, you'll need to edit the following line in /etc/init/bumblebeed.conf (original line prefaced w/ #):
# start on (login-session-start or desktop-session-start)
start on runlevel 
From there, reboot and enjoy. I looked to see if it would be easier to have bumblebeed start dependent upon some other xorg-related service, but I could find nothing. I've also had similar problems with mdm and getting x11vnc to upstart off it. What I'd love to see is mdm reworked to emit upstart events similar to those emitted by lightdm. This isn't a problem with lightdm as default login manager.