Summary
I believe that there will not be long term implications on ATT Iphone exclusivity. Handsets initially released on one network is a long industry practice that has been going on for years. It has not led to domination of one Network operator over the others. Apple would prefer to see the iphone on competitor's networks as soon as possible to help them get a wider distribution and market share.
Analysis
While ATT is reaping the immediate benefit of having the iphone operate on its network, I'm sure Verizon, SPRINT & T-Mobile are anxiously waiting to offer the iphone on their networks. Which puts Apple in the driver seat. I believe the cost to ATT to keep the iphone on their network will be very steep.
This is an industry practice that has been going on for years. RIM Blackberry (first rolled out on Bellsouth Wireless Data), Motorola RAZRs (on Verizon) all were handsets that could only used initially on one operator network. But over time, these devices becamse availability on competitors networks where they had greater success.
And Apple must be looking forward to a wider distribution and accessibility of being on multiple networks.
