I've noticed over time that everyone has the same SMS tone for their iPhone. It's not like the iPhone doesn't give you a lot of choices . . . but it is very obvious that there's only one choice that actually sounds nice and sounds like an SMS. Out and about, I've never heard an iPhone SMS beep that wasn't that iconic one that everyone uses.
I just realised what this is . . . and knowing Apple, I'm quite certain of this. It's branding. Everyone will get to know the sound of the iPhone, because everyone's iPhones beep in the same way. It's not just the iconic look of the iPhone that's become so recognisable . . . now you don't even have to see it . . . you just have to hear one receive a message nearby. On top of that, every time someone gets an SMS on an iPhone, all the iPhone users nearby pull their phones out to check if it's their's.
It's another one of those subtle things that Apple does to make sure that its products are the most visible and the most recognisable on the market.