Comment 2 for bug 515155

The problem is caused by the pam-module pam_ecryptfs.so from package ecryptfs-utils, which tries to mount the encrypted private directory when the screensaver is unlocked.

This seems to be a known problem and is mentioned in /usr/share/doc/ecryptfs-utils/ecryptfs-pam-doc.txt.gz:

Troubleshooting

      Problem: Attempts to authenticate to the gnome-screensaver
      application result in a long pause and a failed attempt.

            Solution: Your PAM stack is not set up correctly. Make
            certain that /etc/pam.d/gnome-screensaver does not point
            to a file in the auth context that includes
            pam_ecryptfs.so.

On ubuntu 9.10 with ecryptfs-utils installed /etc/pam.d/gnome-screensaver includes pam_ecryptfs.so via /etc/pam.d/common-auth.