"Yeah, couldn't possibly have anything to do with the iPod being the best overall PMP, could it?"
Actually, it couldn't. PMPs are commodity components jammed into a plastic case in the far east with some software. Technically products from Sansa, Creative etc are as good, with marginal differences in the user interface. The big difference is that Apple did a first class job of branding the iPod/iTunes link and selling the package. Teenagers are particularly susceptible to that kind of marketing and do tend to follow the crowd, without evaluating boring factors like value for money. That's not a criticism of Apple - with next to no technical differentiation, they found a way to capture the market.