Device side USB port pinout
The IXP420 has a built in USB 1.1 device controller. This is different than the two USB ports provided on the back as it is USB 1.1 and only a slave port. The internal USB controller would be used to connect the NSLU2 to a PC for example. You can not attach other slave devices like hard disks to this internal port.
Allan has managed to get the port working. Hardware and software modifications are needed. It is all detailed in the AddDeviceSideUSBPort howto.