I was looking few days after a working solution to can start my ubuntu machine without monitor plugged. The only one working for me is described bellow and is found here.

Ubuntu Desktop 10.04 is not using anymore the xorg.conf file placed in X11 folder. Create the xorg.conf in /etc/X11 with the following:

Section “Device”
Identifier “VNC Device”
Driver “vesa”
EndSection

Section “Screen”
Identifier “VNC Screen”
Device “VNC Device”
Monitor “VNC Monitor”
SubSection “Display”
Modes “1024×768″
EndSubSection
EndSection

Section “Monitor”
Identifier “VNC Monitor”
HorizSync 30-70
VertRefresh 50-75
EndSection
EndSection

In case you don’t have the privileges to write a new file on /etc/X11 forlder, in terminal use: gksudo nautilus to open the file browser with root privileges.

Then, edited the grub file: sudo gedit /etc/default/grub

This will open the grub file. Now find the line that says GRUB_CMDLINE_LINUX and add “nomodeset”.

You’ll have to end up with something like this: GRUB_CMDLINE_LINUX=”nomodeset”

Now, update the grub from terminal with update-grub otherwise the Ubuntu will be loaded without graphical interface at all.

Restart your machine. Now you can use your desktop without a monitor.