Bug 216951

I have a system where users can log in but
if the screen locks it can never be unlocked.

I am not sure what has happened to the system 
but it is clearly testing for something very strange instead of the
password and failing, without giving the user or even adding
something to /var/log/messages to give any clue as to what the problem
is.

Each attempt to unlock a screen always produces two
entries in /var/log/messages one for the user and one for 
root. But it always fails whether you give the user passwd or
the root password. 

Oct 31 16:05:54 archimedes-304 unix2_chkpwd[12716]: pam_authenticate(gnome-screensaver, shackle): Authentication failure
Oct 31 16:05:59 archimedes-304 unix2_chkpwd[12717]: pam_authenticate(gnome-screensaver, root): Authentication failure

The same password works reliably when logging in 
every other way, (console, GDM, ssh etc)

/etc/pam.d/gnome-screensaver is unmodified. 
In fact 
rpm -V pam pam_modules gnome-screensaver
shows no modified files for any of these packages.