That's pretty neat! I wonder if they will defend you against hostile mobs too, not just attacking the ones you attack.
Only quote I've seen so far from the programmer indicates that they won't attack Creepers. Too big a risk of the whole pack getting wiped out in one explosion.
"The way I see it, every life is a pile of good things and bad things. The good things don't always soften the bad things. But vice versa, the bad things don't necessarily spoil the good things or make them unimportant."