Updating to 10.8.2 has broken virtualbox for me. The error I'm getting is: "VT-x is being used by another hypervisor". I'd be careful updating if you use virtualbox for development.
I tried reinstalling/updating to the latest version of VirtualBox, but it did not help.
Update: I managed to disable VT-x by following the procedure described at http://tech.techteam.gr/virtualbox-vt-x-problem-cannot-disab... and starting the virutal machine with VBoxHeadless. However, this immediately made my host machine completely lock up, so I cannot recommend it :)
Both Windows 7 guest (blech) and Linux 2.6.18 (aka Oracle Developer Days) guest just launched on OSX 10.8.2, VirtualBox 4.1.20r80170, no problems.
Oracle just release VirtualBox 4.2. Release notes mention Mountain Lion support.
http://www.oracle.com/us/corporate/press/1842885
Happy Hunting!
Same Error. mayday