I have a Mac Book Air and wondered why the fan speed and hence the CPU would start to spike every so often.
If I transferred large amounts of data or in reality any amount of data from 100MB to 1 GB to even 7GB to the Air ,away the fan would go.
If you turned the machine off and then restarted, away the fan would go after a while.
I had even gone to the extreme of formatting and reinstalling .
However if you transfer the data and then leave the machine running , not doing anything to it just leave it open and running after a while sometimes up to an hour the fan speed decreases and the machine goes back to normal with no fan spikes.
This is because it takes time for the machine to process all the data you are sending to the laptop.
Remember the Mac Book Air , is basically a " net-book " , slim and should be used for internet browsing and writing , nothing that is too intensive for the cpu.
At least you can here and see when the Mac Book Air is struggling unlike the netbook which just appear's to grind to a halt.
It will be interesting to see how these perform when Snow Leopard comes out as you should be able to get more power out of the mac-book Air with Snow leopard installed.