MacOS X 10.3.2 PowerBook Fan Hack
I got the extensions mixed up. It's AppleADM103x, not AppleADT746x. If you came here on the 20th or 21st of December and the hack didn't work for you, try again with the correct extension. Sorry.
Only do this if you know what you're doing. I'm not responsible for damage to your system, burned palms or exploding PowerBooks. You have been warned.
In MacOS X 10.3.2 a few System Extensions handling the temperature readout and the fan control have been changed. To be more precise, the fan turns on when the CPU temperature reaches 51 degrees C (64 before) and turns off again when it falls below 47 degrees (57 before). As a result, on some PowerBooks the fan does not turn off at all after the update, most notably the PowerBook 12". This might be just a bug, but probably it's Apple's answer to the temperature problems these models have (in hot weather, the palm rest gets too hot to touch).
Downgrade the relevant System Extension. It turned out to be:
/System/Library/Extensions/AppleADM103x.kextThis extension handles the temperature readout from certain Analog Devices Digital Thermometers over the I2C bus.
First, make a backup of the version that is included in 10.3.2.
The old version for the Extension can be found in the standalone updater 10.2.8. Don't run the updater (or quit if it launches automatically).
Extract the extension. To extract single files form the Package, get something like Pacifist or use the shell.
Copy the extension into /System/Library/Extensions.
Make sure that the owner of the extension and everything inside (it's a package and packages are directories) is root and the group is wheel. THIS IS IMPORTANT. Otherwise, the system won't load these extensions, which may have severe consequences. The Finder displays the owner root as system, so this is ok as well.
Enjoy the Silence. (and put on your asbestos gloves again)