Double-click instead of single-click in Ubuntu 12.04

If you've manually edited xorg.conf, try adding and entry for your mouse, in case it was somehow deleted:

Section "InputDevice"
# generated from default
Identifier "Mouse0"
Driver "mouse"
Option "Protocol" "auto"
Option "Device" "/dev/psaux"
Option "Emulate3Buttons" "no"
Option "ZAxisMapping" "4 5"
EndSection

Source: http://ubuntuforums.org/showthread.php?t=1844513&page=2

EDIT As mention in the comments, this seems to be a reported bug that has stood for some time now. It may be best to add your info to Launchpad to help resolve the bug in the future.