/etc/apt/sources.list:

deb http://download.virtualbox.org/virtualbox/debian intrepid non-free

Execute:

wajig install virtualbox-2.1

Answer yes to install the kernel module, it should just work. Create the virtual image:

VBoxManage createvm -name ubuntu -register
VBoxManage modifyvm ubuntu -memory "2048MB" -acpi on -boot1 dvd -nic1 nat
VBoxManage createvdi -filename “/var/vbox/ubuntu.vdi” -size 20000 -register
VBoxManage registerimage dvd ~/ext/debian-40r6-amd64-netinst.iso
VBoxManage modifyvm ubuntu -dvd ~/ext/debian-40r6-amd64-netinst.iso

Start the image:

VBoxHeadless -startvm ubuntu
VirtualBox Headless Interface 2.1.2
(C) 2008-2009 Sun Microsystems, Inc.
All rights reserved.

Listening on port 3389

Connect to the image (requires X):

rdesktop localhost

You can login remotely if you want, or forward the port.