I made some measurements with a script which is somewhere around here in this forum. I modified it a little bit.

The basic idea is to load the bp27x00_battery kernel module, poll the current and voltage on the battery from /sys/class/power_supply/bq27200-0, and calculate the actual watt usage by using that the current is measured internally by the voltage drop over an 22 Ohm resistor.

I have installed the power kernel, I undervolted (but not overclocked), and I activated smartreflex VDD1 and VDD2.

Some results:

Idle with locked (black) display uses around 20 mW.
Idle with brightness 1 uses around 300 mW
Idle with brightness 5 uses around 650 mW
Playing video ("Angry Birds cinematic trailer") with brightness 2 uses around 770 mW

This should all be consistent with my normal usage of the device, where I have around 3-4 days until I need to recharge it. (4800 mWh / 4*24 hours = 50 mW on average)