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I have no idea how to do that, but I'm almost positive you'll have to drop out of objective-c and Cocoa to do it.

Here's an open-source fan control application where you could peek at the source code: http://www.lobotomo.com/products/FanControl/index.html.

And another one: http://www.conscius.de/~eidac/software/page5/page5.html.

One website says:

Please note that Apple does not support an official or standardized way of reading out sensor data in Mac OS X (the only exception is the Server Monitor application for the Xserve series). Moreover, there is no documentation or other technical note about the individual function of the sensors.
 
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