I'm not sure what the true root cause of the problem is, but, when running Windows 8 Customer Preview in VirtualBox, the set of allowed resolutions is pretty pathetic.
I don't even know where / why anyone would buy a 4:3 ratio monitor anymore, so I don't know why there aren't a bunch of supported 16:9 options in here.
In order to get around this, there's a command that, though simple, needs to be done from the command line (after shutting down your virtual machine)
So, if you go to your Virtual Box Folder (usually C:\Program Files\Oracle\VirtualBox) and run the following command, you should be OK:
Then, when you go back into your machine, you should see the new resolution listed for use.
I'm not sure how to make this resolution available in every subsequent virtual machine, but I'm glad it's available in this one.