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I had this problem as well. The kernel_task process was using (in fact just reserving) >100% of the available processor cycles.
My solution was to send it to my local Apple Service Center.
I blogged about the problem and its solution:
I had the exact same problem(s) described in lundbergj's blog. I was about to bring my Air to the Apple store for a new logic board, but before I did, I figured I'd try the latest version of CoolBooks before spending $300 bucks on hardware. I upgraded to 10.5.8, installed the latest CoolBooks, and guess what, my air is running like it never has. I ran OSX + Windows + Word + Photoshop + Safari + played a bunch of You Tube videos, all day... and hooray! no spike in CPU cycles from the kernel_task. Before, I would often experience a CPU spike caused by the kernel_task with just the browser open - sad really.
It's a tragedy that apple is not doing anything about this. Ironically, I removed CoolBooks when when Apple released the firmware update that was suppose to resolve the CPU meltdown issue not long after the first Airs came out. I'm glad I held on to my CoolBooks license key.
Ok I just got my air back, first of all the broken hinge was repaired by Apple without additional costs. But I learned my lesson, as soon as the hinge again has some loose movement the machine is off to the repair center to prevent further damage. The breaking hinge came basically over night, but the display already had been wobbly for several months. It basically broke with a snap by a normal open display operation.
It maybe helped to notice that the problem is a known problem of the rev a series to get the repair under Applecare warranty.
The second issue was my constant heat problem, the guy at the repair shop said that the fan was dirty and clogged to a big degree, he blew it out and cleaned it.
Lets see how things are holding up, I am in the middle of a restore process of about 400.000 files which are restored from an external HD, the time machine restore task has been hammering my machine now for 20 minutes and so far no kernel task is going haywire, it is constantly between 5% and 7%.
So things look good for now. If it holds up, I guess the fan of my machine has to be cleaned in regular intervals as long as AppleCare is still valid.
(probably every half a year or so, due to my heavy usage of the machine day in day out)
And after that I probably will do it myself from time to time.
Ok after three hours of heavy load my air still has a low kernel task.
I am very happy now. Operations which used to trigger the kernel task now work flawlessly.
So in my case it was a clogged cooling vent.
Which means I probably will have the air cleaned out as soon as the problem shows its ugly head.
Flash video did not use to be a problem back then either. The reason for the flash going haywire was that there is an old compatibility lib for old actionscript installed along with the new library, once you delete that flash processor usage drops significantly. Somewhere along these kernel threads there is a direct description regarding which file to delete.
Flash itself is just a symptom not the cause, flash pushes both the processor and graphics card heavily, once a certain point of temperature is reached the kernel task goes up!
The same goes for the logging daemon under certain condition pretty much every application or service which stresses the processor. Those are all normal operations but they are merely symptoms of the cause nothing more.
Actually I gave my Air now one day of extensive treatment, playing flash while doing a backup against an external hd etc...
The kernel task never went higher than 11%
One thing which stunned me was that the processor temperature can get quite high now 85 degress celsius and sometimes 90 degrees on high loads without the kernel task kicking in.
(I assume the better cooling keeps the temperatures on the other parts lower)
I also noticed the fan went one time above 6200 rpm and the cooling down happens much faster now.
Under normal surfing load I am around 54 degrees C now. For now the problem is fixed for me, and it was indeed the clogged fan (clogged from day to day operation I assume, I use the air quite extensively)
And no Flash is not a problem...
I had the kernel_task taking 150% of the CPU problem on my 1st Generation Macbook Air. AppleCare replaced the logic board and fans, and sent the machine back. Problem still occurred. I did a clean install of 10.5.8 and the problem still occurred.
Eventually after 2 months of back and forth with AppleCare and the product support specialists and sending "Capture Data" to their engineers, Apple conceded this was a feature designed to prevent the Macbook Air from overheating.
Making the machine unusable when loaded, is not a feature to me. Eventually I was transferred to customer relations in AppleCare who looked at the support case. They agreed to send me a new Macbook Pro that wouldn't have overheating problems. They said that this "feature" also exists in Rev. C MBA's.
So summary if you are having problems with kernel_task taking up massive amounts of your CPU: This is the software trying to prevent the machine from overheating. Solutions are (a) work in an Apple Store at 68 degrees (b) try running CoolBook <www.coolbook.se> and see if that helps you, it has helped many (c) work with AppleCare to get a new product that doesn't exhibit this problem.
It was a frustrating 2 months, but I'm glad Apple came through in the end.