The PowerBook Pismo (PowerBook G3) has a small cooling fan on it's left side next to the PCMCIA assembly. Most users who have a stock CPU will have never heard, or seen this fan functioning.
The temperature limits for this machine appear to be set incredibly high, and some of the only users who have ever noticed this fan functioning are the ones who have upgraded CPUs. When the CPU reaches the machine's temperature limit the fan kicks in.
 
If you're worried about always running your Pismo at high temperatures the Open Firmware commands may be of use.
 
You can set the fan to run on boot by modifying the thermal-info property.
I haven't figured out how to vary speeds or set multiple temperature points, so if you can improve on this please let me know.
 

Storing values in the NVRAM

 
  1. Boot into Open Firmware by holding Command + Option/alt + O + F when booting your Pismo.
  2. Run the following to edit the contents of the NVRAM:
    nvedit
  3. Run the command below which tells the NVRAM to open the power management device
    dev mac-io/via-pmu/power-mgt
  4. Enter the following to encode new thermal properties. Be careful to note spaces.
    01010200 encode-int 00000000 encode-int encode+ 01010200 encode-int encode+ 00000000 encode-int encode+ " thermal-info" property

    You can use encode-int to encode a single value, but adding encode+ allows you to build up properties. In this case there are four values so we need to use encode-int and encode+.
  5. Press Control + C on your keyboard to exit the NVRAM editor.
  6. To ensure these values persist for consecutive boots, you need to store these values in the NVRAM.
    Note: Any data stored in the NVRAM only persists while the machine is connected to a power source. You can turn your machine off but when it’s unplugged, and it has a flat battery you will need to perform these tasks again. It may be easier to run an Open Firmware script on boot if this is something you wish to run long-term.
    Run the following commands:
    nvstore
    setenv use-nvramrc? true
    reset-all
  7. Your machine will now reboot

You should hear the fans running several seconds into the boot process. If you wish to check the thermal-info properties, boot into Open Firmware and run the following commands:

  1. dev mac-io/via-pmu/power-mgt
  2. .properties

If you find a better way of achieving this, or a way to gain more control please get in touch.

 
Sources
James Little - lists.apple.com