I have an external usb keyboard of which the status leds follow the leds of the internal keyboard of my laptop. So if I press numlock on my laptop, the numlock leds of both the laptop and the external usb keyboard light up.
I meant 'using the Arduino as a keyboard' using the Keyboard libraries (http://arduino.cc/en/Reference/MouseKeyboard
) to send keypresses. I was considering building something akin to this usb-password-typer. http://ob-security.info/?p=631
So I guess catching the led status should be possible via usb, but it might indeed not be implemented in the keyboard library yet.