The point about draining batteries is very pertinent

It happened to me just a few days ago when I shot this: http://www.flickr.com/photos/rawhead...in/photostream

Where, when i whipped out my iPhone (after a 2~hour or so of shooting, which was presaged by me using the iPhone to navigate me to this spot) it was as dead as a doornail. I was VERY happy I had my 5Dmk2 on me to use as a light meter replacement