At the end of the video I do a 60 second test to show how much iron is gathered. On average, I get anywhere from 650-700 per minute.

I plan on designing a 1,000+ per minute farm when I get the time.

World Download: http://www.mediafire.com/?cv3vbpo399uyz33

Details:

This is the most simple, yet productive farm I've designed so far. I've discovered a few main things for making farms:

Surface area matters greatly.
3x3 instead of 2x2 spawn room is much better.
Pistons are less productive than the water currents used.
If they fall 7 blocks below spawn space, new mobs appear.

This would be easy to make as it requires in all seriousness, not many resources. You could disassemble a village and refit it for production of new villagers, using an elevator and minecart system to transport them to the top where they can be led through a tunnel overhead to the falling chamber where they will gather with other villagers in the main holding facility - (2x2x1) center area.