Yesterday I finally upgraded the MacBook from 10.8.1 to 10.8.2 and immediately VirtualBox stopped working. Instead of opening my VMs, I was greeted by the message:
Failed to open a session for the virtual machine Windows XP.Turns out this is a new bug tracked at https://www.virtualbox.org/ticket/10965
VT-x is being used by another hypervisor. (VERR_VMX_IN_VMX_ROOT_MODE).
VirtualBox can't operate in VMX root mode. Please close all other virtualization programs. (VERR_VMX_IN_VMX_ROOT_MODE).
It is fixed using HotFix package 4.2.1 for Virtualbox available at https://www.virtualbox.org/wiki/Downloads