See Headless Virtualbox

When you get say, Windows XP setup in your VirtualBox virtual machine, you can easily forward ports \
from the Host OS, to the Guest OS. An example, I want to use the windows Remote Desktop to remote into \
my virtual XP installation. I open port 3389 on my router to point to my ubuntu desktop, then I forward \
that port into my guest OS.

List vms: VBoxManage list vms
List Extra Data (port forwards): VBoxManage getextradata xpserver enumerate

To forward to a machine
http://ubuntuforums.org/showthread.php?t=433359&page=37

VBoxManage setextradata "NameOfTheVirtualMachine" "VBoxInternal/Devices/pcnet/0/LUN#0/Config/remotedesk/Protocol" TCP
VBoxManage setextradata "NameOfTheVirtualMachine" "VBoxInternal/Devices/pcnet/0/LUN#0/Config/remotedesk/GuestPort" 3389
VBoxManage setextradata "NameOfTheVirtualMachine" "VBoxInternal/Devices/pcnet/0/LUN#0/Config/remotedesk/HostPort" 3389

Lots of CLI options
The VBoxManage Command has lots of options
Like to start a vm from the CLI (though still pops up the window)

$ VBoxManage -startvm "Windows XP"

Try this
http://www.howtoforge.com/vboxheadless-running-virtual-machines-with-virtualbox-2.0-on-a-headless-ubuntu-8.04-server

Multi Monitor
http://forum.virtualbox.org/viewtopic.php?f=1&t=17524