GPIO4 and the USER button: resource busy?

in my application I would like to use the USER button to trigger a shell script. I’m running Ubuntu 10.10 on the xM board rev.C (Linux omap 3.2.2-x4 #1 SMP PREEMPT armv7l)
Apparently in trying to export the GPIO4:

echo 4 > /sys/class/gpio/export

I get a resource busy error.
I can’t understand why the GPIO4 should be in use: the schematic shows it to be connected only to the user button so no other peripheral and kernel module should have it.
On the other hand the GPIO7 which was bound to the user button in previous revisions, is available for export, and I can read it, but it always LOW, proving that the schematic is right.

What could be that prevents me to get privilege to read GPIO4? It is not present in /sys/class/gpio.


P.S: reference page