I've configured the GNOME screen saver to lock the screen on my live-HDD after some period of no activity - it seems to make sense on a mobile platform, to prevent a passerby from messing about with my system. All was well until a recent upgrade (I can almost hear you: "when will this guy get the point?").
The screen saver comes up after a while, but it doesn't lock the screen. The password dialog box doesn't come up when I touch the mouse or keyboard, and I simply get my desktop.
My first guess: I turned off locking by mistake. Easy to check (just launch gnome-screensaver-preferences). Nope. The screen saver is definitely set to lock the screen.
My next guess: it's a bug. After a quick look at the gnome-screensaver bug page on the Debian BTS, I found bug #481119. While the reported problem isn't quite similar to my own, the bug submitter provided a workaround that seemed worth a try.
I opened up gconf-editor, found the key /apps/gnome-screensaver/lock_dialog_theme and modified its value from default to an empty string. A longshot. I know. But it works. I would never have guessed.
I'll go wait for the screen to lock now. Bye.
[Aug. 19 2008] UPDATE: I posted too early. It sometimes works and sometimes doesn't, and I can't quite put my finger on it. In the meanwhile I've reverted the value of /apps/gnome-screensaver/lock_dialog_theme to its previous default value default.