Are you implying that the iPhone/iPod touch would take the place of the iPad, and thus one would't "need" the iPad? Because there is a
monumental difference in the way one interacts with an iPad compared to how one interacts with an iPhone or iPod touch. So just because someone who owns an iPhone goes out and purchases an iPad doesn't mean they don't need it the iPhone could
never serve as a replacement for the iPad. The iPad is definitely more like a computer, compared to the iPhone which is more of well... a phone.
That being said, one of the huge selling points that
Apple is making about the iPad is that "you already know how to use it." It's advertising to people who have iPhone and iPod touches because it runs the same operating system. Sure, there are going to be people who go out and buy an iPad without any experience on how it works. But there are going to be considerably
more people who are going to buy the iPad who already
do know how it works. Why? Because that's what people are comfortable with. Because iPhone/iPod touch users are already comfortable with using those devices, purchasing an iPad is going to be more likely. And that's what Apple's betting on too which is why the iPad runs like an iPhone, and not like a Mac.
Think of it like this: the reason why people still buy PCs. They know that Macs are better, but they're not willing to make the switch because they're just
used to Windows. They're not willing to make the jump until they become
comfortable with it, or until someone shows them. So the same would go for iPhone/iPod touch users they're
used to the iPhone OS. They're already
comfortable with it. So they'll stick with the iPad because they're accustomed to the iPhone OS.
(The difference is that the iPad rocks while Windows sucks, lol).