Watch a 90 minute youtube video over 3G and you're pretty much there. (On my iPhone 3G at least)
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.
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.
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.
In contrast, a phone that requires charging every week, for example, you'll tend to keep at a higher total charge and will survive periods of heavier usage better.
That's not dismissing the fact that most people are willing to accept the tradeoffs of modern smartphone battery life, but make no mistake the necessity of daily recharging for average use is a problem.