This post, Blackfriars' Marketing: Why a million iPhones in 74 days is better than you think, has a lot of good reasons why the iPhone will likely meet it's goals.
If you need more reasons, how about it's very likely that within the next year (probably right before Christmas, about a year from now), the iPhone will get revved with a lot of things holding people back (3g, possibly a developer SDK or least several officially supported partner developers, etc...). Those would still count as iPhones in Jobs' mind I suspect. They are going to add -more- value to the phone over time, which will help sales.
And second, the iPhone I suspect has a network effect being discounted. For one, people think it's cool. The phone sells itself to a large degree. More people out there with them, means more selling opportunities. I haven't met anyone who after playing with it wasn't at least interested enough to consider it.
And second, this thing is going to change people's idea of the mobile web. More iPhone compatible mobile sites will spring up, making it progressively more useful. Clearly Apple is hoping that effect will be powerful enough to cause people to go away from things like Flash for primary content and go to web standards/ajax.
So quite simply, all other bits of the sales curve aside, the phone will get cheaper and better over the year, and that will help sales I suspect.
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