This guide explains how you can run virtual machines with VirtualBox 4.0 on a headless Ubuntu 11.04 server. Normally you use the VirtualBox GUI to manage your virtual machines, but a server does not have a desktop environment. Fortunately, VirtualBox comes with a tool called VBoxHeadless that allows you to connect to the virtual machines over a remote desktop connection, so there's no need for the VirtualBox GUI.