I think most people would agree that the iPhone 4S is an evolutionary upgrade over the original iPhone. Indeed, many, many of the Enhanced Browser/No Keyboard/App Store Enabled/Touch Screen smartphones today are closely related to the iPhone in a manner that that iPhone is not related to the Palm Treo - that's all I'm trying to say - that Apple did something completely new with the iPhone, rather than just iterating on a previous design.
I will _never_ forget the day I used the iPhone of a friend (I was among those who thought the concept of Apple getting into the "Phone" business was ludicrous, and was determined not to get one) - and used that browser/map. Within 48 hours I had an iPhone, and never looked back on my dear Treo.
Just because a particularly successful combination of traits looks like a giant leap doesn't mean it is (consider things like Nobel Prize winners or Olympic medalists born to "ordinary" parents). Similarly, the relationship of the iPhone to its predecessor phones is tighter than you think. Almost all of the traits that made the iPhone what it is were present in the predecessor population of smartphones.
In particular, I'd say the iPhone is what you get if you cross two specific devices and add the mutation of capacitive multi-touch (and the inevitable march of Moore's Law, of course). Those two devices are:
1. the O2 XDA (full touch screen, no keyboard, "enhanced" browser with desktop-like ambitions [even if it was Pocket IE]) 2. the Danger Hiptop (consumer focused device with similar built-in apps, centralized app download catalog / store)
Those ancestors were chosen quite carefully, of course. The manufacturer of the O2 XDA was HTC. A bunch of ex-Danger people (most notably Andy Rubin) went on to start Android. A smartphone cross in the iPhone's immediate vicinity was going to happen regardless.
Did the iPhone do the things my Treo could do much, much better? Was it a great step forward? Absolutely. The iPhone was and is amazing [1]. Nevertheless, saying that the iPhone stands apart from the broader evolution of mobile devices is either ignorance or a willful misreading of the relevant history.
[1] Though I will admit to finding the idea of paying money for ringtones of music you already owned a repugnant step back.