Update: So it turns out iPhones *do* have a way to turn it off but its just that I at least never read the manual (because you don't need to) and didn't discover it. My main point is still interesting I think, that you don't need it enough to want to find it. As usual I will leave my mistake here for the world to read. :-)
Apple get usability. By now there must be very few people who don't realise that. But in case there are some out there, lets just look at a couple of examples that struck me recently.
I own an Apple iPhone. I bought it in San Francisco, unlocked it myself, and have been using it constantly for the last month. It is without question the best phone I have ever used. Not perfect, for all the (3G etc.) reasons that you can read about on the blogsphere, but it is pretty damn close, and certainly closer than any other I have owned, including my abortive attempts to get to like "smart phones" in the past (Palm, Nokia etc).
But it struck me after a month of using the thing that there is a glaring missing feature. In fact it is so obvious I'm astounded that I haven't noticed it before. There is no 'off' button.
There is a button on the top right that you press to lock it - the screen turns off but little more.
There is also a 'be quiet' button on the left side, above the volume control. Hit it, and the phone gives a little shake and then remains silent. This is equivalent to choosing the 'Silent' profile on a Nokia or other phone.
The most amazing thing about the lack of an 'off' switch is that you never notice it. Hey, it took me a month. And when do notice, and start to think about it, you realise that you never actually needed an off button anyway. The only reason that I used it on my old phone was because I was going into a meeting, and I wanted to make sure I wasn't disturbed.
But this shows the reason that Apple doesn't need it. With my old Nokia, 'Silent' was a profile, and I had to trust that the people who created that profile did the right thing, and turned all the sounds and notifications off. That they didn't think "You know, this feature that I'm working on is really *really* important -- no-one would *ever* turn it off -- so I'll give an ever so subtle beep even in Silent mode". I don't trust anyone to do that. I don't trust *myself* to do that if I'm developing software.
So what Apple does is say: "No, this isn't an option, its not a profile that you can customise. No questions asked -- hit this button and I won't disturb you. Not at all. Never". It is still a trust issue. But I trust that button. I trust it because it doesn't feel like a software option -- the button implies a hardware solution -- a built in switch that kills the sound. (Of course I understand that in reality it is software, but trust is conveyed by the visceral physical click of that button).
Apart from wanting to make it silent, why would you ever turn off your mobile? These days turning off a mobile is like locking yourself in a dark room and calling out to your friends that you aren't coming out. Life -- or at least social life -- stops . So the Apple iPhone has no off button.
Another shorter example. I am writing this sitting outside the Apple store in NY. You know the one, the glass box on 5th Avenue. Every couple of minutes someone takes a photo of it. It is beautiful, ad the plaza that surrounds it is comfortable and popular and full of people sitting eating lunch. A shop that gives something back. Nice.
Inside the store, it is very busy, very crowded. Apple stores have the highest per foot profitability of any store here. But it is still cool and comfortable and a pleasant place to be. But it is quite noisy. So how do you sell iPods, which are 'all about the music' as Steve Jobs likes to say in a noisy environment?
I don't know how other companies would do it, but what Apple do is load all the iPods on display with Bose sound cancelling headphones. These are the $250+ headphones that you see long distance travellers (like me) using on long haul flights to cut down the airplane noise and get some sleep . Despite the noise in the store you really can hear the music, and the chaos of the store disappears when you put them on.
This is the attention to the user experience that has made Apple what it is. It is pleasant to visit an Apple store. It is pleasant to buy an Apple product. It is pleasant to use an Apple product. And it is why the Apple iPhone doesn't have an off button.
 While writing this it struck me that the same is true of my laptop. I never switch it off, I just close the lid. And that makes me wonder if we will someday see a laptop from Apple that has no off switch?
 They are using the AC3 ones that sit on the ear, and I guess that is because a used over the ear headphone might feel unsanitary, whereas the on the ear design leads to less actual contact. They are not the battery operated ones though, I don't know what exact model they are.