Right and I found I do need it - I must have typed the parameter once for fun, and there was my trial-and-error scientific method out the window; because I was trying to choose between the hypotheses of hardware or user error, I didn't care about acpi, whatever it is.
VirtualBox provide downloads for earlier versions, and at some stage you do get to choose whether you want automatic updates. So it would be possible to downgrade.
Although the documentation looks thorough at first glance, I actually still found it hard to set everything up; and you really do need the "Additions" to make it usable for more than playing with. You now need the kernel source installed inside the guest to add the additions. Don't know if they are also still needed in the host. Anyway, I found the package somewhere in /var by doing
and even though the shared folder feature doesn't work yet without the Additions, you can get the guest to look at a usb stick (though I don't think it can be mounted by host and guest at the same time).
I am writing from a Ratpoison guest inside a Ratpoison host, and having different escape keys seems like a good idea. Well, for the moment the guest is still C-t, and the host's Menu escape key works wherever the cursor is. I don't know what would happen if host and guest used the same escape key. Menu is a good one, because pressing it twice gets you back where you were, and that didn't work for me with Super-L or -R.
But then, why would anyone want an identical guest and host?
