I have Mid 2012 non Retina MBP and I have the same problem. I applied the pre-release 10.8.2 software update and VB stopped working. Nothing I try seems to help.
Same problem here (VT-x is being used by another hypervisor.), I'm on a freshly installed retina with 10.8.2. A lot of people are going to have problems as 10.8.2 rolls out during the next couple of days....
Same issues here - 10.8.2 Retina Macbook Pro -- VirtualBox is hosed. Need a fix for this ASAP -- can't do work until this works. Really will be annoyed if I have to restore the entire machine from backup.