Comment 1 for bug 419829

Thanks for your report. Here is some information as to where inhibit went:
http://blogs.gnome.org/hughsie/2009/01/28/inhibits-and-the-new-world-order/

Basically all inhibiting is done with gnome-session-manager. The dbus interface is described in the gnome-session documentation in /usr/share (it's a dbus call to gnome-session-manager.inhibit()). The command you are referring to was a python script written specifically for Ubuntu, which was broken when inhibit was removed from GPM.

Someone will either have to write a new script and possibly place it in either gnome-session or GPM, or you could use the gnome inhibit applet. You can add it to the top panel, clicking on it should inhibit shutdowns/suspends. There are some bugs with that applet, but we are hoping to work them out soon.