I noticed a while back that the VirtualBox OS window is set to transparent in some cases when Compiz is also running (a lot of users have reported this, though it's not happening for everyone).
Compiz brings a lot of bling to the Linux desktop, but with such an extensive selection of configurable features, it can sometimes be difficult to figure out how to take advantage of specific functionality. Several readers have asked me how to use Compiz to make menus and tooltips transparent, so I figured I'd share this trick with the Open Ended audience.
By default Compiz in Ubuntu uses the GTK window decorator. You can adjust the transparency level of the GTK Window Decorator window titles by changing a value in the GNOME configuration editor. Note that this only works if you are using a compositing window manager such as Compiz.