The sell of the iPhone depends on its performance. Is it stable? How many bugs there will be? The quality of Apple’s product has decreased dramatically lately. If the iPhone’s performance is poor (as MacBooks’ first generation was), the phone will sell around two million units. After that the reputation is ruined and the product will vanish from the markets.
If the iPhone is as superior as Apple hope it will be then the device could sell easily 10 million units in a three years. But Apple has to introduce new models every three months or so. One or two models won’t do the trick.
So, my prediction is that the original iPhone will sell two million units in a 16 months (because of buzz). After that we will see is it going to be new iPod or Newton.