There is a fine and noble defense against AoO + Cleave, though it does take a little reading between the lines. There are three key points, two of which are clear, and one which is only alluded to in Tome and Blood under familiars.
1. AoO for movement is only provoked when LEAVING a threatened square.
2. No AoO is possible when the target has 50% or more cover.
3. From page 18 of Tome and Blood, "it can occupy the same space as you, enjoying one half cover".
So, the first opponent stops in the first threatened square. The second opponent moves through the first opponent's square, is immune to AoO while so doing, and stops in the first UNOCCUPIED threatened square. And so on, with the opponents 'leapfrogging' each other to surround the target without ever provoking an attack of opportunity. It's called 'tactics'.
Now, this requires, in my mind, that the attacking creatures be intelligent, organised, and actively act to provide cover for each other as they advance. No unintelligent beasts or chaotic rabble is going to be able to pull it off. You should have seen the look on the face of the psychic warrior with spiked chains, Stand Still, and Combat Reflexes when I pulled this on him. And, before you ask, each of the individual rules 1-3 above had come into play at some point so he was well aware of them all. He just had not foreseen how they could be used in combination.
If you don't buy this defense, well there's always Spring Attack, the bane of all creatures who depend on reach attacks.