I have:

* Ubuntu Gutsy, but previoulsy was Feisty with the same issue.
* A multihead setting, where my laptop is the :0.0 and 
  a external LCD display (using VGA connection of my 
  notebook) is :0.1.  Driver i810.
* Screensaver preferences are:
  * Regard the computer as idle after 10 minutes.
  * Activate screensaver when computer is idle.

If I work continously in :0.1, after a while (10 minutes) both
screen starts a fade out, having the same behaviour if the
computer was idle, but it wasn't idle.  This happens even if I'm
use the mouse over :0.1.

When I see the fade out, I just move the mouse from :0.1 to :0.0.

The expected behaviour is the screensaver should start the fade
out only when the computer were actually idle.  Without activity
in all displays.

Other information:
* Power Management:
  * Put display to sleep when inactive for 40 minutes 
    (on AC Power, which I know is one of my use cases).
  * Put display to sleep when inactive for 15 minutes
    (On Battery Power).