Ubuntu v10.x will sometimes freeze during boot with a black screen under VirtualBox. I have observed this under VirtualBox 4.0, but it has been reported to occur with other versions of VirtualBox.
The fix is easy. Turn off the affected virtual machine, then use the following command to disable ACPI in that VM:
vboxmanage modifyvm <vm name> --acpi off
Restart the VM, and it should boot normally. (If the VirtualBox directory isn't in your path, you'll need to CD there to use vboxmanage).