Dangerous "memory effect"

This one is hard to describe without a digital camera to take a picture...

My video card is a Radeon 4850HD. Since Oneiric, right before logging in, my computer exhibits a strange and very dangerous "memory effect"... Most of the time, after entering my credentials to login, a screenshot (the memory effect I mention in the summary) of a previous session displays on the screen for about 10 seconds. Many times, this is not a screenshot of the last state of the screen prior to logout, but a seemingly random shot that occured at some point during the session. There are also many more visual glitches (corruption) prior to log-in, compared to Natty (which was practically flawless). So, I am unsure whether to blame this on the open source Ati drivers (also occurs with proprietary drivers) or LightDM (a new variable which seems likely to blame).

This is an extremely dangerous bug. I recently logged in, with a friend behind me, and while watching my screen, she saw about 10 seconds worth of a work passwords file displayed clearly on the screen, even though my encrypted partition was not even mounted, nor was the partition containing this file left open prior to reboot.

I have ruled out hardware issues, since switching back to pre-Oneiric releases and Debian releases fix this.

Having chatted with a fellow Ubuntu user on IRC, I found he exhibited the same issues. He had the same concerns as me, but seemed way less worried. Since I found no previous bug report (could be my own faulty searching), I felt obliged to report this.