This is an interesting breakdown on laptop power consumption from the Windows Tips and Tricks UPDATE newsletter:
Q. What uses the most battery power on my laptop?
A. The LCD display. In fact, almost half the power in most laptops is used by the screen, which is why dimming your screen can save you a lot of battery life. Below is a list of the major power consumers on your laptop. This list also applies to desktop computers, other than the screen figure.
Hard Drive: 5%
Other bits: 10%
Obviously different systems will vary based on their components, but this gives a rough idea of where that battery power goes.