I've copied / slightly modified this from this ArchLinux bug report which is affecting multiple people: https://bugs.archlinux.org/task/47151

Description:

Disabling switchable graphics does not work since upgrade to xorg-server-1.18.0

With the xorg upgrade the instructions to disable the switchable graphics (https://wiki.archlinux.org/index.php/NVIDIA_Optimus) do not work anymore.

Xorg Server version with bug: 1.18 in all cases (1.17 works fine)
nVidia driver versions tried: 358.16, 361.18


Xorg configurations tried and result:

1)

Section "Module"
Load "modesetting"
EndSection

Section "Device"
Identifier "nvidia"
Driver "nvidia"
BusID "PCI:4:0:0"
Option "AllowEmptyInitialConfiguration"
EndSection

# Result:
# The undesirable effects I personally noticed within a few seconds are:
# - dmenu broken: it doesn't show up all the time, most likely on another screen I can't see (I'm using i3, starting X with a startx)
# - running Steam will popup 1/4 of the login window on the top left of the screen
#- games ran by steam such as CS:GO will be extremely confused by the screen layout and you are simply not be able to play

2)

Section "Module"
Load "modesetting"
EndSection

Section "Device"
Identifier "nvidia"
Driver "nvidia"
BusID "PCI:4:0:0"
Option "AllowEmptyInitialConfiguration"
Option "nvidiaXineramaInfo" "False"
EndSection

#Result:
# firefox broken (left click not working, UI half dead, ...)
# reportedly: qt4 apps broken, eclipse broken

3)

Section "Module"
Load "modesetting"
EndSection

Section "Device"
Identifier "nvidia"
Driver "nvidia"
BusID "PCI:1:0:0"
Option "DPI" "96 x 96"
Option "AllowEmptyInitialConfiguration"
Option "Coolbits" "31"
Option "RegistryDwords" "PowerMizerEnable=0x1; PerfLevelSrc=0x2222; PowerMizerDefault=0x1; PowerMizerDefaultAC=0x1"
Option "nvidiaXineramaInfoOverride" "1600x900+0+0, 1600x900+1600+0, 1600x900+0+900, 1600x900+1600+900"
EndSection

#Result:
# Almost works.
# Fullscren window apps are a bit broken, they don't respect my tint2 bar (everytime behind it... now I use autohide option to pass this).
# Setup is particular to a specific configuration / monitor.

If you follow the link at the top of this report to the ArchLinux bugzilla, there is also an example Xorg.1.log and xorg.conf for download if needed. I've instead included other xorg configurations in this list as they were more inline with the nvidia recommendation as mentioned here:  http://us.download.nvidia.com/XFree86/Linux-x86/358.16/README/randr14.html 

Can reproduce this if needed to collect further information, just ask! In the meantime I am running xorg-server-1.17.

Thanks.