Another is the way the cell phone industry works in the U.S. While AT&T's wireless unit may have well over 60 million subscribers, only potentially 2.5 to 5 million of them may have contracts expiring around the time the iPhone is released, which limits the number of people who can buy it. Sure, some will probably ignore any additional costs of upgrading before their contracts expire, but that number will be relatively small.
This also applies to subscribers who have contracts with other cellular network operators. The likelihood of large batches of subscribers jumping from their current carriers for Cingular also is probably small.
Then there's the $499 and $599 price tags. Most cell phones sold in the U.S. go for $100 or less. Enterprise-class smartphones, which can run a bevy of third-party productivity applications, sell in the $200 to $400 range. Many of them have media capabilities such as video players, cameras, and music players.
With such fierce competition lined up against it, you'd think Apple would be making the iPhone as attractive to as many people as possible (with lower prices and greater availability). Apple is, however, charting its own course.