Touchpad hotkey does not work on ASUS P81IJ

$ sudo evtest
[...]
Select the device event number [0-10]: 6
Input driver version is 1.0.1
Input device ID: bus 0x19 vendor 0x0 product 0x0 version 0x0
Input device name: "Asus Laptop extra buttons"
[...]
Testing ... (interrupt to exit)
Event: time 1412379709.628840, type 4 (EV_MSC), code 4 (MSC_SCAN), value 6b
Event: time 1412379709.628840, type 1 (EV_KEY), code 530 (KEY_TOUCHPAD_TOGGLE), value 1
Event: time 1412379709.628840, -------------- SYN_REPORT ------------
Event: time 1412379709.628856, type 1 (EV_KEY), code 530 (KEY_TOUCHPAD_TOGGLE), value 0
Event: time 1412379709.628856, -------------- SYN_REPORT ------------
Event: time 1412379717.081469, type 4 (EV_MSC), code 4 (MSC_SCAN), value 6b
Event: time 1412379717.081469, type 1 (EV_KEY), code 530 (KEY_TOUCHPAD_TOGGLE), value 1
Event: time 1412379717.081469, -------------- SYN_REPORT ------------
Event: time 1412379717.081488, type 1 (EV_KEY), code 530 (KEY_TOUCHPAD_TOGGLE), value 0
Event: time 1412379717.081488, -------------- SYN_REPORT ------------
^C
Above is the result I got after two (Fn + F9) strokes on my ASUS P81IJ laptop.
The hotkey does not toggle the touchpad as intended.

Additional informations:
$ lsb_release -rd
Description: Ubuntu 14.04.1 LTS
Release: 14.04

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: udev 204-5ubuntu20.7
ProcVersionSignature: Ubuntu 3.13.0-36.63-generic 3.13.11.6
Uname: Linux 3.13.0-36-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.5
Architecture: amd64
CurrentDesktop: Unity
CurrentDmesg:
 [ 310.062530] IPv6: ADDRCONF(NETDEV_UP): wlan0: link is not ready
 [ 1511.112582] init: acpid main process ended, respawning
 [ 1513.124409] init: acpid main process (2696) killed by KILL signal
 [ 1513.124438] init: acpid main process ended, respawning
 [ 1932.872956] perf samples too long (2510 > 2500), lowering kernel.perf_event_max_sample_rate to 50000
Date: Sat Oct 4 06:42:59 2014
InstallationDate: Installed on 2011-10-18 (1081 days ago)
InstallationMedia: Ubuntu 11.10 "Oneiric Ocelot" - Release amd64 (20111012)
MachineType: ASUSTeK Computer Inc. P81IJ
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-3.13.0-36-generic root=UUID=00ab2e2c-ccf1-4f9d-a430-a4623ac322f4 ro quiet splash acpi_osi=Linux acpi_backlight=vendor vt.handoff=7
SourcePackage: systemd
UpgradeStatus: Upgraded to trusty on 2014-04-20 (166 days ago)
dmi.bios.date: 02/14/2011
dmi.bios.vendor: American Megatrends Inc.
dmi.bios.version: 206
dmi.board.asset.tag: ATN12345678901234567
dmi.board.name: P81IJ
dmi.board.vendor: ASUSTeK Computer Inc.
dmi.board.version: 1.0
dmi.chassis.asset.tag: ATN12345678901234567
dmi.chassis.type: 10
dmi.chassis.vendor: ASUSTeK Computer Inc.
dmi.chassis.version: 1.0
dmi.modalias: dmi:bvnAmericanMegatrendsInc.:bvr206:bd02/14/2011:svnASUSTeKComputerInc.:pnP81IJ:pvr1.0:rvnASUSTeKComputerInc.:rnP81IJ:rvr1.0:cvnASUSTeKComputerInc.:ct10:cvr1.0:
dmi.product.name: P81IJ
dmi.product.version: 1.0
dmi.sys.vendor: ASUSTeK Computer Inc.