From the manual (in the dmg):

- Can I damage my CPU if I run it at a lower voltage?
- No.

Known issues

No long-term testing has been performed on OS X 10.5 (Leopard).

The CPU of the "Santa Rosa" / LED MacBook Pro can be read and controlled using CoolBook, and the maximum power consumption may be reduced.
There is however an issue, making this model consume more power running in idle battery mode with CoolBook activated.
CoolBook is therefore not recommended for usage with this model.

yes this app does work, undervolting is a well known concept on the windows/linux side of town, unfortunately this app is terrible. I say this as someone who registered, coolbook is ridiculously limited.

My main gripe is that using while using coolbook you are locked into the multiplier that you have modified. Coolbook prevents the CPU from ramping up when needed.

My second gripe is that it does not work with core 2 duo imacs although the hardware is capable.

pps macman (above) is the author of the program

The best way to get your MacBook to run cooler. The fan noise is no more.

Wow, it acually works as advertised, although I don' quite understand how the adapter/battery pref pane toggle works, really good results though, with two instances of yes > /dev/null going peak temp is 69 degrees at 1.67GHZ (I have a 2 GHZ core duo macbook pro) when undervolted to 0.95 compared to default of around 74 degrees at a voltage of 1.1625. Main selling point for me is increased battery life, I get 2 hours when maxing the CPU now!, idle battery life is unaffected though, still around 4 hours.