Comment 19 for bug 444495

A quick work-around is to change the line in /etc/grub.d/30_os-prober at ~line 28 that says
  if [ "x${found_other_os}" = "x" ] ; then

to

  if [ true ] ; then

Probably the better method would be to create a new option in /etc/default/grub so it's easy to enable the menu if dual booters wish to see it by default,

Add the following to /etc/default/grub
GRUB_FORCE_HIDDEN=true

Then in /etc/grub.d/30_os-osprober
find the line
  if [ "x${found_other_os}" = "x" ] ; then

(around line 28)

and change it to
   if [ "x${found_other_os}" = "x" ] || [ "x${GRUB_FORCE_HIDDEN}" = "xtrue" ] ; then

Which checks if GRUB_FORCE_HIDDEN is true, otherwise continue with the default behavior. This allows you to re-enable the menu by changing GRUB_FORCE_HIDDEN to false if you wish to see the menu again.