One day I'll learn that when prompted to upgrade something, the safest option is almost always 'no' (especially, I might perhaps unfairly add, if it has anything to do with Oracle). The latest version of VirtualBox has broken VBoxVMService and it's now impossible to run the service and the VirtualBox GUI at the same time. If the service is running then trying to open the GUI now gives you an error
Failed to create the VirtualBox COM object. The application will now terminate.
Callee RC: CO_E_SERVER_EXEC_FAILURE (0x80080005)
A real pity.
Details in the VBoxVMService wiki: Running VirtualBox GUI together with VBoxVmService.
Update: I reverted to VirtualBox 4.1, but VMs wouldn't start, moaning about VERR_SSM_UNSUPPORTED_DATA_UNIT_VERSION and the like. A bit of googling and the solution is just to right-click on each VM and choose 'Discard Saved State'.