Gnome 2.30 (Ubuntu 10.04) renamed the DeviceKit.Power to UPower:

$ echo 'dbus-send --system --print-reply --dest=org.freedesktop.UPower /org/freedesktop/UPower org.freedesktop.UPower.Hibernate' | at now + 30 minutes

There’s also suspend.
Shutdown (Stop) and Reboot (Restart) works over the system call: org.freedesktop.ConsoleKit

Or use this tool

—

Pre Gnome 2.30

As of Ubuntu 9.10, the gnome-power-cmd doesn’t exist anymore..but…
D-Bus to the rescue!!

Hibernate after 30 Minutes:

$ echo 'dbus-send --system --print-reply --dest=org.freedesktop.DeviceKit.Power /org/freedesktop/DeviceKit/Power org.freedesktop.DeviceKit.Power.Hibernate' | at now + 30 minutes

There’s also suspend.
Shutdown, reboot and logout works over the session call: org.gnome.SessionManager

Or use this tool

—

Pre Ubuntu 9.10:

Suspend:

$ gnome-power-cmd suspend

Hibernate after 30 Minutes:

$ echo 'gnome-power-cmd hibernate' | at now + 30 minutes
job 1 at Mon Sep  7 15:10:00 2009

You can review and delete any queued job with:

$ atq
1	Mon Sep  7 15:10:00 2009 a wag
 
$ atrm 1

The gnome-power-cmd also accepts shutdown and reboot.