Thats a great tip! Lets try this app!
I have been using this app for a few months now, and it has not let me down yet. For those saying it needs a 'spot' function, it has something very similar, in that you can hold the phone steady with your shot framed, and tap the screen to meter different areas in the frame.
I also like that it shows readings as EV, Lux, and FootCandles (although I only use EV).
It's a little slow to react compared to a conventional light meter, but hey, it's a free and fairly accurate light meter on my phone!