Latest updates to Arch Linux include Gnome 3.6. With this, CosoleKit is replaced by logind. This requires the system to boot with systemd.

https://www.archlinux.org/news/consolekit-replaced-by-logind/

Previous announcements indicated this was coming.

https://www.archlinux.org/news/systemd-is-now-the-default-on-new-installations/

I use xinitrc to start Gnome and found that startx no longer worked. Going through the recent announcements and subsequent pleas for help by others in the forum that inevitably start showing up after a change like this, I discovered how to fix my problem. Simply adding "init=/usr/lib/systemd/systemd" to the boot parameters in Grub. The process looks like:

Go to /etc/default/ and bring grub up in an editor. Add or edit the line GRUB_CMDLINE_LINUX= with:

GRUB_CMDLINE_LINUX="init=/usr/lib/systemd/systemd"

It's possible there are other parameters on this line so you'll need to separate them with a space. The whole line should be enclosed in "".

Then run grub-mkconfig to generate a new grub file with the new settings:

grub-mkconfig -o /boot/grub/grub.cfg

Then reboot and use startx normally and it should work. This was the solution for my issue. References include:

https://wiki.archlinux.org/index.php/GRUB2
https://wiki.archlinux.org/index.php/Kernel_parameters
https://wiki.archlinux.org/index.php/Systemd
https://bbs.archlinux.org/viewtopic.php?id=151776&p=1
https://bbs.archlinux.org/viewtopic.php?pid=1184402