The problem of MacBook Air first generation overheating seems to be sloppy heat engineering on Apples part. After my machine started to exhibit the usual behaviour again (fan on full speed, kernel task pid 0 with 160% CPU load) I took it apart, which is surprsingly easy. You only need a very small phillips screwdriver and a bit of good thermoconductive paste or pad (available in usual PC shops). Remove the back plate and you will immediatelly see the fan and heat sink. Remove all the dust you see, especially around the fan. Then unscrew the heat sink (the black piece of metal) and carefully lift it up. Carefully remove all dust and all grey goo (the thermo-paste) from the chips and the heatsink without smearing it across the board. Replace old thermo-paste with a smaller ammount of fresh paste (or better some cut to size thermo-pads as used on modern systems). Carefully put all screws back, and make especially sure you get a tight fit between chips and heatsink. Replace back plate. Download some CPU temperature measurement tool and check your success. At first, the CPU temperature will go up to over 90°C, then the thermo-pad / paste will be “burned in” and make close contact between chip and heatsink. Temperature will lower again and afterwards things are nice and cool. At least as far as I can say after two days.