I've set up a prototype; it uses a 1U power supply (suitably modded so that it runs even though it isn't powering a motherboard), a couple of 12v fans, a 12v ethernet switch and a 12v relay for cycling power to 8 things. I had to make adapters from the 1U power supply to power the ethernet switch and relay; they expect to be powered by wall-warts, but they seem happy enough to get their 12 volts from the 1U power supply. Then I measured the power draw.
The PSU, without doing anything, takes 88 ma. I hadn't expected that - it puts a floor under the whole thing. The ethernet switch takes 30 ma, the fans take about 10 ma each, and the relay box about 1ma. Pies will pull 500 ma at 5v, which is 10 ma at 240. So a dish with 8 Pies will draw 230 ma, and a dish with 16 will draw 310 ma. This should be compared with a conventional server with 6 hard drives, pulling 440 ma. The dish will still need at least one conventional server, because I can't see how to attach hard drives to a Pi (I know I could use the USB port, but I have a feeling that this will put too much burden on the Pi's USB system).
Checking the power supply, it says that it can do 12 amps at 5v, and another 10 amps at 12v, so the power supply will be very lightly loaded by 16 Pies. It's a switching PSU, of course, so it should give *very* stable power.
Just one fly in the ointment - I only have one Pi. The other ten are supposed to arrived December 18, although I'm not optimistic about that.