Binary package hint: emerald
I don't know if this bug ticket belongs to gnome-screensaver or to emerald. Since it does not happen when Metacity is the window manager, I must assume that it's emerald's problem.
I often have two users logged in at the same time via GDM. When I switch between them, it automatically locks the screen (even when I try to configure it NOT to -- one user is a dedicated MythTV login, and I don't want that to screenlock). When I switch back to a login where Beryl and Emerald are running, the gnome-screensaver dialog appears, but I cannot type into it. I have to go to a linux console, log in, and kill the gnome-screensaver to get back in. If a terminal or editor were the last focused windows, I can see what I tried to type into the unlock dialog.
(Another annoyance; probably the subject of a second bug report; is that if I use the logout dialog to switch user, I must enter the password at the GDM screen, and then again at the gnome-screensaver unlock dialog. Can't Dbus be used for this or something?)
I have an issue with compiz on Feisty that I think may be the same thing.
When my laptop is suspended, the screen is locked, when it wakes it asks for the password. If, before I manage to type my password, a network connection is set up, then a notification from nm-applet very briefly flashes on the screen. At this moment the password dialogue loses keyboard focus, and no amount of clicking on it will give it back, and I'm forced to alt-ctrl-backspace to restart X. An obvious workaround is to not plug the network in until I've entered the password, but this doesn't help if it auto connects to a wireless network quickly.
I haven't attempted to see if waiting for the notification to go away helps at all, I just noticed the connection between the notification and the losing focus tonight.