I'm probably on the opposite extreme (never have watched a movie or tv show on my iphone nor any youtube video longer than about 3 minutes), but I'd be shocked if long video watching was a common case.
Battery life on any iPhone sucks the big one. Hell, it sucks on any smartphone I've ever seen. Big, power-hungry touchscreen phones are simply not in a place where we can afford to use them all day.
I sit down and play Mirror's Edge for 10 minutes and out goes 10% of my battery. Holy crap.
But even taking gaming out of it, even moderate browser usage over 3G burns through that battery like kindling. I have to charge the iPhone at work if I plan on going out in the evening. Even with judicious limitations on usage (and isn't that defeating the point of a smartphone?), if I leave with a phone at 9am, go somewhere after work, there is a good chance I won't have enough juice to call for a cab at midnight.
I don't have a problem with a 24-hour charging cycle, my problem is that current smartphones - iPhones and all - have trouble lasting that full 24-hour period unless you severely handicap your own use of the device.
Not sure but I did it once because I had a lot of time to kill waiting for my girlfriend to finish up at work.
Another annoying scenario is when you lock the screen but keep a high CPU app running. Go back and resume a couple hours later and your battery is decimated.
I'm probably on the opposite extreme (never have watched a movie or tv show on my iphone nor any youtube video longer than about 3 minutes), but I'd be shocked if long video watching was a common case.