Due to license restrictions the new Mac OS X as Guest feature is only available to users with Apple/Mac hardware, and only for "Mac OS X Server" installations as a guest OS. Any other combination is unsupported and will therefore not work. Any post that talks about a different combination will be removed without warning. Any discussions of ways to get around the Apple SLA will also be removed.
Snow Leopard Server sla --- Snow Leopard sla
At this time VirtualBox is not officially supporting the Mac OS X Lion operating system as a virtual guest and there is no time-line when it will be officially supported. According to the released SLA from Mac you should be able to install Lion but the same restrictions apply as with Snow Leopard including Mac hardware, Mac OS only. See the link below.