About your power calculations, I think you overestimate the power draw of a "full" computer. I tested around my house using a Kill-a-Watt to measure power draw at the outlet. The workstation I'm writing this on is a dual-processor "core i7" Xeon 5520 system, with Geforce GTX 260, a 15krpm SAS drive as primary OS and six 7200rpm WD Black SATA drives. Its draw at the outlet is 200w idle.
(EDIT: Also 24GB of DDR3 RAM... Plenty of room to reduce the system wattage well below 200w.)
I have an Athlon XP system that has two hard drives and discrete graphics, clocks in at 150w. With a different case, it would be RAID capable.
I'm sure a low-power PC could be built with standard components. It would not get as low as the DS1010+, but the power bill would not be as dramatically high. A laptop with eSATA may actually come close to the DS1010+ power savings while still providing the server flexibility of a full computer.