It might have allowed for 12cm fans, but it you had looked, you would see that they are using 10 much smaller fans. Ick.

Meanwhile, the x4600 (8 dual core Opteron system) does apparently use 2 12cm fans.

With all those disks, I suppose it might not make much difference, but I would have rather seen them using 12cm fans on the x4500 as well.