How do I disable X at boot time so that the system boots in text mode?

For Ubuntu 11.10 and higher
Edit /etc/default/grub with your favorite editor,

sudo nano /etc/default/grub

Find out this line:

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"

Change it to:

GRUB_CMDLINE_LINUX_DEFAULT="text"

Update Grub:

sudo update-grub

No need to remove / disable lightdm upstart conf, it already does that for you.
lightdm.conf

# Check kernel command-line for inhibitors, unless we are being called
        # manually
        for ARG in $(cat /proc/cmdline); do
            if [ "$ARG" = "text" ]; then
                plymouth quit || :
                stop
                exit 0
            fi
        done

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You will still be able to use X by typing startx after you logged in.

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