Description:
With the installation of xf86-input-synaptics, a file is installed to /etc/X11/xorg.conf.d/10-synaptics. The presence of this file seems to cause a double loading of the synaptics driver, sometime leading to a non-functional touchpad:
https://bbs.archlinux.org/viewtopic.php?id=104769

Additional info:
* package version(s): 1.2.2-2
* config and/or log files etc.

/etc/X11/xorg.conf.d/10-synaptics.conf
Section "InputClass"
Identifier "touchpad catchall"
Driver "synaptics"
MatchIsTouchpad "on"
Option "TapButton1" "1"
Option "TapButton2" "2"
Option "TapButton3" "3"
EndSection

Steps to reproduce:
1. install xf86-input-{evdev,synaptics}
2. start x