This is my current desktop in the office:
As you can see, KDE4 taskbar on the right, windows 7 taskbar on the left. Windows from the right can move onto the left screen freely (note the linux terminal on the left screen). Unfortunately due to limitations in the virtualbox seamless mode windows from the left screen (from windows 7) are limited to that screen. Also, there’s no fancy aero 3D effects.
Still, it’s a nice way to run windows and Linux side by side.