[Full-Disclosure] Xfree86 video buffering?

[KF (lists)]

Recently I have noticed that after shutting down my machine or rebooting 
my X-windows will briefly flash an image of whatever I was doing when I 
rebooted the machine or logged out.

As an example if I was browsing porn at night in mozilla just before I 
clicked "K->Logout...->Reboot Computer" in KDE, the next morning when my 
Girlfriend turns my computer on she gets a quick 2 second shot of my 
porn  just before the KDM login screen.

What causes this? Can it be prevented? I can certainly see sensitive 
data potentially being leaked via this quirk.
-KF