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.
Same Error. mayday
Again, isn't that the point?!
I've had both, and I'd rather have to plug it back in occasionally than have my $2k MBP smashed on the floor.