Wait, what?
This isn't some custom repo or anything, I'm just running stock Arch and have startx in my bash profile. It runs in user mode, and has for about the last half year.
https://wiki.archlinux.org/index.php/Systemd/User#Xorg_as_a_...
Unfortunately, to be able to run xorg in unprivileged mode, it needs to run inside a session. So, right now the handicap of running xorg as user service is that it must be run with root privileges [...snip...] It seems likely that xorg could be modified [...snip...], and then it could run unprivileged from a user service outside a session.