This would do what I think you want:
http://www.iomega.com/direct/product...rd_id=12196655
But it still needs mains power, I guess power over ethernet will come but then you need yet more kit.
I guess if power consumption is a real issue you might save a bit of power using this but that needs to be balanced agaiinst cost.
Have you thought of using Wake on LAN ?
Most modern network card / PC motherboard combinations allow a PC to be remotely switched on over the network. So if you are working on PC "A" and need a file on the drive of PC "B" you just send a "magic packet" to PC B , wait for it to boot and access the file. IF you use VNC and you can then remotely shut "B" down. Obvioulsy would work B to A too.
Assuming you have the right hardware (reasonably likley -if not even a cheap network card can do this.) save for the set up time this option is free.
Best regards
Memetic.