I tried summoning a Killer Rabbit by using the following command inside a command block, triggered via button:

/summon Rabbit ~ ~1 ~ {RabbitType:99,Attributes:[{Name:generic.attackDamage,Base:30}]}

The rabbit spawned and proceeded to attack me. I expected the Killer Rabbit to be somewhat comparable in power to the Monty Python version, but alas it was not.
The Killer Rabbit only dealt a measly 1 heart (2 pts) of damage while I was wearing a full suit of Diamond. Base 30 should be dealing 60 hearts (30 pts) correct? At best, the Killer Rabbit can only manage 6 hearts of damage on an unarmored player in Hard difficulty, regardless if it's attack base is changed to 30, 100 or 1000.

Steps to Reproduce:
1. Start a game in Creative mode with cheats enabled
2. Give yourself a command block using "/give @p command_block"
3. Place command block on ground
4. Right click command block
5. Type the following into the field provided:

/summon Rabbit ~ ~1 ~ {RabbitType:99,Attributes:[{Name:generic.attackDamage,Base:30}]}

6. Place a stone button on the command block
7. Press the stone button to activate the command block
8. Go watch Monty Python and the Holy Grail because it's one big let down from here on.