Interface reply, and Battery Life reply
An early poster, Paul F, stated that interface alone will sell it. It is certainly true that the interface and stylings of the iPod sold it in droves, despite it being inferior to alternatives, more expensive, limited to iTunes DRM, and having battery life problems. I'll reserve judgement though on the iPhone, as mobile markets are more established, especially in the East and in Europe, and people are fussier about looks and interface versus content. Also, a number of people have mentioned tha lack of tactile return on typing is offputting, so I'd have to use one before I comment fully, but my early guess is that no, it will sell well based on style and interface, but because it's a more technical device (a phoen and multimedia) it will NOT sell anything like the iPod. Apple have spent that one.
Battery Life - someone asked about the complaints on this, so I'll explain as noone else has. The early iPod was heavily criticised as battery life was limited - not only was it much shorter than most players (possibly the drain caused by the touchwheel), but atfer a lot of charges, your battery dies. Unlike other devices, apple priced the replacement battery so high that buying a new iPod was necessary. So after 2 years, your iPod would have to be rebought. Batteries have improved since, and since then thrid parties have fouind ways to provide replacements - although opening it invalidates any remaining warranty.
With the iPhone, the same issue is resurrected. Almost every mobile phone has a battery compartment, which is easily accessed allowing you to replace the battery, and the likes of Nokia, Sony Ericsson, etc sell batteries seperately at a fair-ish price. This allows travelling users to swap batteries rather than wait for a recharge as well. Apple have gone down the same old road as the iPod, with an interface that drains power, meaning lots of charges fast, and no way to swap out the battery, so once it's capacity is diminished, you would have to buy a new phone.
So if you buy a new phone every 2 years, not too big a deal, otherwise, limiting. Bad design.