Since I installed the b271 update earlier today, I can't seem to find a sheep anywhere. It took me minutes to find a chicken. Ordinarily, grass-covered ground on my server is pretty much choked with friendly mobs of all types. You can't walk anywhere without seeing small herds of friendly mobs. But no more. Is this related to the "stop the mobs from leaking into other worlds" bit - are they being culled in error? Is anyone else suffering this? I'll try reloading the server without any mods if this is really not happening to anyone else, but I suspect it's simply that nobody's thought it was noteworthy. Alternately, was the earlier profusion of friendly mobs a bug?
I've noticed the same thing since adding bukkit a few days ago. It seems to only truly affect friendly mobs, but it might be affecting both.