"This guide explains how you can run virtual machines with
VirtualBox 4.0 on a headless CentOS 5.6 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."