As I understand it, all the US carriers (with the exception of T-Mobile) charge the same per month whether you bought a phone subsidized or not, so it doesn't make too much sense to buy an unsubsidized phone, because you won't be saving anything per month.

Fair point on what is "mainstream". I was thinking in a US-centric way, I admit. Here the two best selling phones are iPhones (as far as I know).

But after thinking about this more, I just don't think it is helpful to analogize to the iPod. The difference in markets is huge, and I doubt the iPhone's share will ever be like what the iPod's was (and still is).

What you're really saying is that the total cost of owning the device is more than just the purchase price. I agree -- so how would an iPhone nano help?