What causes a Macbook Air (mid-2011) crazy CPU cooling since upgrading to Mountain Lion?
I had similar on my late 2008 MBP 15" after upgrading to Mountain Lion.

I found a post about resetting the SMC controller (system management controller) to solve this and it worked a treat for me. I have a removable battery on my MBP so the process was a little different to unibody mbp's where you can't easily get at the battery. It was basically:


remove power cable and battery,
hold the power button for 5 seconds.
Power up normally and see if it worked. 


For portables without a user-serviceable battery (like your Macbook Air):


plug in the power cable to mac and a power source,
press the (left side) ⇧ ⌃ ⌥ keys and the power button at the same time,
release all the keys and the power button at the same time.
Power up normally.


Give it a try - there are plenty of posts on the web on how to reset it. Mine is quiet as a mouse now