I’m sure since the last OS X Lion Service Pack (I forget what the slick Apple term is for “here’s a shedload more code to fix stuff” is) that the kernel_task process just takes off on it’s own. 40% CPU? 800mb RAM? Sure, help yourself. It must be very important it’s job whatever it is otherwise Apple wouldn’t let it work this way would they? Between it and mdworker it’s amazing that anything else runs. It is supremely annoying to have other processes judder to a halt because of this.