I've got a MacPro w/ a 23" cinema display running 10.5.5. I've had this problem since 10.5.2.
In System>Prefs>Display>Options, when I set the display to "Put System to Sleep or Wake Up", and then lock the prefs, my computer randomly unlocks the Display prefs panel and resets the option to "Turns the Display On/Off".
I've tried fixing permissions but that didn't make any difference.
Anyone else having this problem and have an idea how to correct it?