When I boot Ubuntu 12.04 in VirtualBox, I see VirtualBox's menu bar at the bottom of the screen. After I log in, the menu bar disappears. Why? I really want it back.
Yes, it appears to be a bug in VirtualBox; it only occurs when you're using Unity hardware accelerated. If on the login screen you choose "Ubuntu 2D", the menu will be visible again.
The menu appears to be drawn under the 3D accelerated window - if you have multimonitor on a Windows host, you'll see that the Windows Taskbar also doesn't show if you change focus to another window while leaving VirtualBox in fullscreen mode on the main monitor.
I'm not sure if the bug is in VirtualBox or the host's display drivers.
Is it in scale mode?
Host + C is the toggle for scale mode (where Host is Right Control by default)