I've been using gnome-screensaver with awesome for a while without any problems. Unfortunately that all came to an end when GNOME 2.26 hit Debian Sid last week. Just so that no-one else has to dig for this, gnome-screensaver now uses gnome-session to determine idle time. gnome-screensaver will run without gnome-session, but the screensaver and locking mechanism will never kick in. Fortunately, there is an easy fix. I changed my
~/.xsession and everything works now.