[ltp] cannot access acpi interface

[fa-ml]

Hello list,

        I recently bought a Thinkpad X60s [1] and I am in the process of
installing software/customising it to my own needs.

I wanted to make some leds blink upon receiving mail, so I started by
probing the acpi interface

    $ root@x60s:/proc/acpi/ibm# cat led
    status:         supported
    commands:       <led> on, <led> off, <led> blink (<led> is 0-15)

Reading this I tried:

    $ root@x60s:/proc/acpi/ibm# echo "15 on" > led
    bash: echo: write error: Operation not permitted

Has anyone of you encountered the same problem? Is there a workaround?
I suspect the solution lies in booting the kernel with a particular
parameter (alas, I did not find which one).
Thanks in advance
-F

[1] http://shop.gluglug.org.uk/product/ibm-lenovo-thinkpad-x60-w-coreboot-wo-extras/