FarmVille pigpens are starting to arrive on farms as you read this. I've already started getting requests from materials from my neighbors on my Facebook wall.
Here's what we've learned about the pigpens so far:
Like the chicken coops, you can only have one per farm
Pigpens can be harvested every 24 hours
When you harvest, you have a 15% chance of acquiring an item
Pigpen takes 8 Wooden Boards, 8 Nails and 8 Brick to build
The pigpen holds 20 pigs
The pigpen is 7x6 or 42 squares