As a general rule of thumb, most Virtualization packages do not support running another such package inside it, it is like trying to install XP inside a running XP, that will not work either.
There are some situations where this is possible with VBox and that is what this post is for, clearification, confirmation and information about the how, why and why not.
You CAN run Virtualization inside virtualization.
What you can't do is use Virtualization Extensions (AMD-V, VT-X) inside a virtualized server.
Do make sure you mention VBox version number and other environment settings/hardware should you have a success story. It should be obvious discussions about running xen inside vmware will be removed as this is the VBox forum.