That's exactly what happens anyway if the MacBook isn't doing anything. The clock speed drops, and the voltage drops. And if the MacBook is working hard, then dropping the voltage would be counterproductive because it would slow it down. The battery would last longer, but you would spend the extra time waiting for the MacBook to do its work.
That's not necessarily true.
Keep in mind that the Intel Core 2 lineup overclocks very well because at its stock voltage, the processor can actually have its FSB kicked way up and still be stable. For example, my current quad Q6600 is stock at 2.4GHz with a 266 fsb and 9x multiplier. Stock voltage is 1.3125 volts. I can crank the fsb to 333 and put my quad core at an even 3.0GHz without even touching the voltage. This is because stock volts is more than enough to cover the original speed - in fact, its actually excessive.
Recent AMD energy-efficient CPU's have shown dramatic ability to undervolt from stock volts and retain speeds. As in things like 1.1 V's being able to run the Athlon 64 X2 at stock speeds despite stock votlage being 1.35V for instance.
So what I think he wants to do is drop the volts permanently while keeping the speed stock, meaning you can run the computer undervolted *all* the time while maintaining the speed, which is doable easily w/ a BIOS but that's obviously not an option here.