Power consumed by an LED in a simple circuit
9V Battery, 4 x 10k Resistors and a single LED, how would I calculate the power consumed by this LED?


You have to know the forward voltage of the LED. A typical number is somewhere between 1.5 and 3 V. Let's say it's 2 V. 

Then there is 7 V across the 40 kOhm resistor, so the current flowing is 0.175 mA.

0.175 mA x 2 V is 0.35 mW.

(Note that typically the forward voltage is specified with 5 or 10 or 20 mA flowing. With only 0.2 mA, the actual forward voltage through a real device will be lower, and so the power will be lower. To get a more accurate estimate would require a more detailed model of the LED behavior)


  is it really as simple as P=V*I?


Yes.