It seems the HAL or PolicyKit package are broken at the moment and fail to install a certain policy file.
Quick fix: cd /usr/share/PolicyKit/policy && sudo wget
http://cgit.freedesktop.org/hal/plain/policy/org.freedesktop.hal.power-management.policy
See also:
http://bbs.archlinux.org/viewtopic.php?id=58047&p=1