I have an HP laptop with Dual ATI/Intel video card. Everything was installed and working with Gnome 3.4.2. I had to use the hybrid-video-ati-intel package from the AUR.
I decided to give Gnome 3.6 and the new 10/6/12 installation media a try so I wiped everything. I followed the Beginner Guide to get it reinstalled and accidentally loaded up Gnome 3.4.2 again instead of Gnome 3.6 from gnome-unstable. But it worked and all I was able to see everything.
I added the systemd kernel options, then the system would no longer boot. After a bit of googling I found that I needed to run
"systemctl enable multi-user.target"
However Gnome and GDM will not start.
Can you start xorg at all?
# pacman -S xorg-xinit $ startx
I can Xorg starts perfectly.
Same with gnome 3.4.2 it works fine.
Looks like the OP discovered a solution.
Possible he needed to
systemctl enable gdm.service