Here is the story with MS Intellimouse Optical series:
If your mouse is connected to PS/2, the light will stay on, even although computer has been powered off. You will need to completely disconnect the power in order to light to turn off
If your mouse is connected to USB, the light will turn off when you turn off your computer, put in standy mode or hibernate.
Also, it doesn't matter what kind of motherboard you're using or what BIOS settings you're applying, statements above are true for every situation.