I have the same problem like mentioned in this thread.
When virtualbox is running in fullscreen mode, and I'll minimize it to go back to Unity, the only visible elements are the wallpaper and the background shadow of the upper bar. If then I click on virtualbox main window return all normal. I want to report a bug, but I don't know if is Unity or Virtualbox issue. Thanks in advance!
I think it exists since unity update yesterday (or day before yesterday).
Any ideas? If you need more information tell me please.
I use ubuntu 12.04 host an Win7 guest.
Graphics: Intel® Sandybridge Mobile