I strongly suspect that the vast majority of people--even here--"remember" a number of Apple's biggest wins as being inevitable from Day One. In fact, as you say, the iPhone didn't become a must have smartphone until 3G/3GS timeframes. Similarly, until the 4th gen iPod with the click wheel, the iPod was pretty much just another reasonably well-regarded, albeit pricey, MP3 player.
I think that anyone that doesn't remember the iPhone as being a wild crazy success was just not paying attention - and the numbers back that up.
The 3GS was when it exploded but it was already a big success by that point.
What people have to remember is the success of the iPhone is practically unprecedented in terms of adoption. It's incredibly rare to come out and dominate an industry like that when you're a new entrant into the space.
Even if the iPhone had continued it's July-September sales curve it would have been a success. It just got even more-so when the other versions came out.
I think we're quibbling over terminology. As you say, it was the 3GS that "exploded." (And it's when I personally upgraded from a Treo 650.) I agree that, to a significantly greater degree than the iPod, the iPhone was from Day 1 obviously a big deal even if it wasn't clear that most competitors really needed to get busy working on their gravestones.
