A vulnerability has been found and corrected in gnome-screensaver:

gnome-screensaver 2.14.3, 2.22.2, 2.27.x, 2.28.0, and 2.28.3, when the
X configuration enables the extend screen option, allows physically
proximate attackers to bypass screen locking, access an unattended
workstation, and view half of the GNOME desktop by attaching an
external monitor (CVE-2010-0285).

The updated packages have been patched to correct this issue.