I've had 2 motherboards that did this (MSI Pro 2A, ECS KT7VZA, and Asus P5A(this board would only do it when the "power up from keyboard" feature was turned on)), when the mouse was hooked up through the ps2 ports it would never shut off. As pamf mentioned all ATX motherboards are always powered up as long as the power supply is plugged in. It seems some also keep the ps2 ports powered up as well. I never found anything in my BIOS to disable it, so I just left it be.