I had the same problem (only took some time to find about it because I did not restart X for a few days). The solution was the same posted at the user-testing list:
I had to include again to XF86Config-4 the section Section "InputDevice" Identifier "Generic Mouse" Driver "mouse" Option "SendCoreEvents" "true" Option "Device" "/dev/input/mice" Option "Protocol" "ImPS/2" Option "ZAxisMapping" "4 5" EndSection and the line InputDevice "Generic Mouse" to section "ServerLayout" Vitor.