I have a 2 week old 24" iMac running Mac OS X 10.6. It is the primary monitor with the menu bar on the top of display. I have an external monitor as well, a 19" Viewsonic LCD. The LCD is set to the left side of the iMac, rotated 90 degrees CCW and has the Dock along the far left edge.
When I restart the screen brightness on the iMac reverts to Full brightness. The Viewsonic LCD retains the setting I have for it. I am using a Mini DVI to DVI cable for the external display.
I even tried setting my Huey Pro to do automatic screen adjustment based on ambient lighting but the iMac still goes to stun with a reboot.
I am sure it is something dumb I have overlooked.