Western Digital My Book Pro Edition II WDG2TP10000 1TB is on clearance at Harris Technology — $499. It has 2 drive bays with 500GB HDD each and does RAID 0 stripping or RAID 1 mirroring. It also sports Firewire 400/800 and USB 2 interface.
HT seems to be the cheapest for this particular model from a brief comparison at staticICE and ShopBot. However Western Digital also has "My Book Essential" and "My Book Premium" with 1TB option as well and at a cheaper price.