I've noticed my cattle disappearing and thought it was other players killing them since we started using the latest snapshots (13w22a-13w26a). I decided to do a test on SSP and SMP by building a new farm and putting livestock in it, then monitoring it. Within a few minutes, the cattle seem to be pushed into the walls and suffocate. Same happens if I isolate cattle to a cell each. They still somehow suffocate in a wall without being pushed into it.