yeh, I believe it's to do with USB power mostly. If you have a device plugged in which has the ability to wake the computer then it interprets the loss of power as... erm, something and tries to wake up the computer. As you can see I can't remember the specifics.
If you have a usb keyboard for example then you can as a workaround edit the power settings in device manager to disable its ability to wake the computer.
However, I have no USB devices plugged in and it still wakes up after about one minute, not worked out how to get round it.
I did manage to get it to sleep properly once but for some reason it no longer will.
My brother said there is a jumper on his asus mobo (not the same one as yours , but the same socket edit: not sure why I wrote that, mind must have been elsewhere cause you don't have an asus mobo or an intel) which lets it turn off usb (or something) which fixed it for him.
