Hey,
doing a netinstall from archlinux, I don't get the shutdown/sleep/reboot button to get "un-greyed" in xfce despite following the late FAQ entry : https://wiki.xfce.org/faq#session_manager

Sample :

[otyugh@depannagePc ~]$ file /usr/lib/xfce4/session/xfsm-shutdown-helper
/usr/lib/xfce4/session/xfsm-shutdown-helper: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 2.6.32, BuildID[sha1]=a1a860083e592eb08ccf2cc1acb838dd74c54e90, stripped

[otyugh@depannagePc ~]$ sudo cat /etc/sudoers | grep %users
%users ALL=NOPASSWD:/usr/lib/xfce4/session/xfsm-shutdown-helper

[otyugh@depannagePc ~]$ groups
wheel power users


And still nothing different : http://imgur.com/v88Bm9il.png

Does it means that xfce does not use sudo anymore ? Didn't gave a try to the dbus solution yet.
I'm quite open to "investigation in real time", I'm up on freenode #xfce most of nights if you want to get me, will be my pleasure.