Comment 42 for bug 150846

As far as I can tell, the reason the "shut down dialog" appears instantly the second time the "Off button" is pressed is because pressing it the first time, and waiting for it to stop hanging, starts gnome-power-manager:

Before pressing the "Off button":

rune@rune:~$ ps -aux | grep power
Warning: bad ps syntax, perhaps a bogus '-'? See http://procps.sf.net/faq.html
rune 7034 0.0 0.0 3004 752 pts/1 R+ 07:12 0:00 grep power

After the "Off button" has been pressed, waiting for it to finish hanging, and pressing "Cancel" in the "shut down dialog":

rune@rune:~$ ps -aux | grep power
Warning: bad ps syntax, perhaps a bogus '-'? See http://procps.sf.net/faq.html
rune 7074 12.0 0.7 23460 7804 ? Ss 07:13 0:00 /usr/bin/gnome-power-manager
rune 7077 0.0 0.0 3004 764 pts/1 R+ 07:13 0:00 grep power