The background would be the windows workspace - you could have linux windows on top of it, though.
It doesn't *seem* all that hard to implement, it would just be like removing the title bar, the window borders, and resizing the thing to exactly the space of your desktop - minus the KDE or gnome taskbar, but, I might be wrong.
This seems like a fun way to use virtualbox, though, and would be my preferred way to do things.