It's possible this is not a "51% attack" but something much simpler, bringing down a pool as large as deepbit, slush or btc guild this close to a difficulty change will 1, decrease the number of new blocks found slowing the flow of new blocks into exchanges theoretically temporarily stablizing the prices(this would happen at any point a major pool went offline however) and 2 cause the difficulty drop to be longer than normal by affecting the blocks/hour ratio even if done for a day this would cause a greater than predicted difficulty drop and allow said attacker to mine more coins than prior durng the next cycle