I can`t solve your problem but I`ve noticed the same on a Fujitsu Siemens Scaleo 600 system. This particular one has a Gigabyte mobo fitted and, like your Abit, power remains at the usb connections.
Another Scaleo 600 with an MSI mobo shuts of the usb ports and equipment with no problems.
As the mobo requires power to enable Wake on Lan, Wake on Ring, Timer start-up etc, I checked the BIOS to ensure these were all disabled. It made no difference on the Gigabyte but could be worth checking on yours just in case.