I found this forum thread to be very helpful:
The solution that I found that worked consistently was post 30 by iscatel:
well, this disables sound, as a previous post pointed out, but here goes:
/etc/default/grub, comment out (by placing
# at the start of the line)
this will pass "text" to the kernel at boot time
line, disabling gdm.
if it still does not work, you can
echo "manual" > /etc/init/gdm.override
and try it again. I used Ubuntu 12.04 32-bit server with GNOME core and it worked for me.