Locking your Screen with dbus from the command line in Gnome, KDE4 and others.

dbus-send --session --dest=org.freedesktop.ScreenSaver --type=method_call \
--print-reply /ScreenSaver org.freedesktop.ScreenSaver.Lock

This will work with KDE4 and should as well work with newer Gnome versions. Tested with KDE 4.1 beta1. It needs the display variable to be set and be run as the appropriate user.
e.g. su $user -c "DISPLAY=:0 dbus-send... "

In Ubuntu 8.04 you can change your /etc/acpi/resume.d/90-xscreensaver.sh to lock the screen on resume like this:

#!/bin/sh

# now, we should poke xscreensaver so you get a dialog
#if pidof xscreensaver > /dev/null; then
for x in /tmp/.X11-unix/*; do
displaynum=`echo $x | sed s#/tmp/.X11-unix/X##`
getXuser;
if [ x"$XAUTHORITY" != x"" ]; then
export DISPLAY=":$displaynum"
su $user -c "(xscreensaver-command -deactivate)"
su $user -c "dbus-send --session --dest=org.freedesktop.ScreenSaver \
--type=method_call --print-reply /ScreenSaver \
org.freedesktop.ScreenSaver.Lock"
fi
done
#fi