You clearly don't know what you're talking about.
Touchscreen phones didn't take off until Apple made one.
MP3 players didn't take off until Apple made one.
Pretty much, Apple shows others "how it's done", then they follow suit.
Nothing has even began with tablets. How many people have YOU seen walking around with those devices that you mentioned. Not many, I bet.
Touchscreen phones were becoming more and more popular. It's simply that the iPhone brought a whole new level of public attention to them. So in that regard, the iPhone did help to drive the market.
MP3 players were popular long before Apple released the iPod. Not sure where you're coming up with that.
I don't think it's necessarily a case that Apple shows "how its done", but rather they carefully see what's slowly trending towards higher popularity, and improve upon it, and it becomes popular (partly due to the fact it's an Apple product). Don't get me wrong, Apple does do an excellent job with many of its products, but reading these forums would make someone think that Apple makes no mistakes and that every Apple product is wildly successful in its market (AppleTV anyone?)
And regarding the issue with a Tablet, you kinda made KnightWRX's point for him. How many people have you seen with a tablet? I've seen one. Generally you're paying a lot more for considerably lower performing hardware, because so much of the cost is associated with the screen. Thus, you either have to have a lower-performance system, to try and cut costs wherever possible, or otherwise price it fairly high and thus it becomes a niche product (basically it'll be the Mac Book Air all over again, only to a greater extent). Given Apple's lust with high profit margins, it's very unlikely you'd see them release anything but a mediocre-performance tablet, probably something like an iPhone with a large screen. If that were to happen, a netbook will sadly still be more desirable.
No, Knight's point is that Apple usually has entered into markets that were already popular or that are becoming popular (MP3 players and "smart phones"), they improve upon existing ideas and release their own product to much fanfare. Since the tablet computing market is fairly underwhelming at the moment, why would Apple enter it? Part of the reason why the iPod became successful was because it took an already popular concept, and made it easier for people to use (via the combination of the iPod and iTunes). With smartphones, which were also becoming popular, Apple once again improved upon existing designs in a market that was growing, and released what was, at the time, essentially a superior product. But the tablet PC market is neither wildly successful nor quickly growing in popularity.
Now with the netbook market, Apple could make a splash. But as has been mentioned numerous times previously, it's a market Apple has no desire to enter.