There is one very VERY important thing you are forgetting.... the underlying server-side tech that allows the visual voicemail and other network services is something that only providers can do. Apple found a partner that was willing to make the infrastructure changes. Give that some time to finlter into the "want" of the marketplace, and other carriers will come around... and THEN the iPhone will be available on all the networks. This is a combination of revolution and evolution, and other carriers - especially VBerizon - have publicly said they were not willing to re-write their systems to deal with one phone. And in response to the iPhone demand... Verizon just issued a new architecture change for their voicemail and data systems. It will take a year to do it, but they have started. 20 months from now, you can have a Verizon iPhone. Just watch.
7 Reasons Why Keeping the iPhone Locked Down is Stupid and Arrogant