It's not the frequency that "wastes" battery life, but the voltage. Normally higher frequencies need higher voltages - more heat is released, the fans have to work and so on.
CPUs are not the same when they come out of the factory - they all receive voltage that 99% of them work with. A percentage of them will work well with lower voltage.
Some of them are sold as ULV (Ultra Low Voltage CPUs) that are expensive to buy because they have better performance for less wattage and less impact on battery life. You have to check your particular processor.
One application that was nice for Mac OS X is CoolBook. It easily undervolts your CPU if you want the frequency.
Undervolting is safe. It's not the same with overclocking or overvolting. When you try to run with lower voltage at 2 GHz for example, you will get to a point where your computer will shut down.
So the ideal voltage will be a little higher than that point.
To give you an idea: my CPU with a nominal frequency of 2 GHz normally runs at 1.2625 V. But it actually runs well at 2 GHz with just 1.0750 V. This difference has a huge impact on battery life and heat, resulting in a lot more battery life without losing performance.
As technology evolves, the factory sweet point for the majority of CPUs is more personalized. But you can still optimize a lot!