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....
I just installed 10.8.2 and my VMs are working just fine in 4.2.0. I'm on an Mid 2011 iMac with a Quad Core i7.
There are three kinds of people in the world. Those that can count, and those that can't. 27" iMac 3.4 GHz Intel Core i7 32GB NVIDIA GeForce GTX 680MX 2048 MB OSX 10.10.3 MacBook Air 1.7 GHz 8GB Intel HD 5000 OSX 10.10.3 System76 Kudu Professional 2.5GHz Intel i7-4710MQ 16GB RAM Ubuntu 14.10
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.