Output Power Meter

Has anyone tried putting a meter on their amp to show output power? I have a voltage meter that I was playing with, but I the calibration with that will be a pain because power is non linear, plus speakers are not a constant impedance. I know there are power meters out there that meaure both current and voltage simulaniously.

Anybody tried this before?
Are you wanting an actual analog meter, or would an LED display do? Analog would be cooler looking (admittedly subjective), but LEDs respond faster.
sound.au.com has a clipping display and, I believe, a power display--there are others out there, but for the life of me, I can't remember where I saw them.
Keep in mind that, to a close approximation, either voltage or current will be proportionate to the instantaneous power, so you can use either one. (It's not worth worrying about V and I getting out of phase due to reactive loads, because the difference would be swamped by the lag time in response of an analog meter, or the separation between the power measured by individual LEDs.) I would recommend monitoring the voltage, as measuring the current would involve either a sense resistor in series with the output, or an inductive probe like some meters use to measure current (which could arguably influence the sound). In lieu of a separate sense resistor, you could use the emitter/source/cathode resistor as a sensor. I can't think of a reason why you would want to read both current and voltage since they're both going to be proportionate, but it wouldn't be hard to do--then combine both through a summing network into an opamp. That would leave you to determine the relative weighting between the two.