I see it more as the result of inevitable cost reductions in recent technology (fast/low power CPUs, capacitive screens, better LCDs, higher density batteries/flash/RAM)...combined with a willingness to tell the phone carriers to suck it and do a phone the way Apple wanted to do it.
Alas, it was not to be, yet. But then it did happen.
Even Jobs has wanted to make a Dynabook from the early days (http://www.folklore.org/StoryView.py?story=Pirate_Flag.txt).
There was also the crew from UIUC that won Apple's "Design the Computer of the Year 2000" back in 1987. Their design? A tablet.
Star Trek TNG also featured imagined tablets, and they started out roughly around the same time. The design just makes sense, it's natural.
But I'd wager the vast majority of people would have still expected a new device using those components to have software full of "computery" things such as scroll bars (probably operated via a stylus) and a task manager instead of the iPhone OS's fluid, natural, free-of-computer-administrative-debris interface.
That's what made the iPhone appeal to people who would have never even considered any other smartphone that was available before it and that's what seemed to come out of nowhere.
Modal interface, nothing that needed a stylus, intelligent button centering, usable gestures. It all added up to a very usable package. The stylus-based phones were just so unusable to me.