Here is my take on the technology...
Apple signed a contract with the devil. The devil's name was AT&T. They have somewhat of a saturated 3G (3rd Generation) network in the larger metropolitan areas, but almost invisible anywhere else. And even in big cities like New York, LA, and Chicago AT&T had one of the worst customer satisfaction records possible. This is problem one.
But all who looked at this contract was sure that AT&T provided a fantastic legal kickback to be able to sell exclusively the most advanced phone in existence.
Now AT&T uses a technology called GSM (Global System for Mobile) for their 3G network. This is a fairly universal technology that is used around the world. So you can make calls just about anywhere, but you will only be able to use the broadband access required for any apps where a true 3G network exists with the available bandwidth. This returns the issue the the first problem noted.
Now the other cell providers have founded their networks on another technology called CDMA (Code Division Multiple Access) for their 3G networks and found that the technology can provide a higher bandwidth without much more monetary investment in existing networks. Just some tweaking and firmware downloads and if the internal network can handle the higher bandwidth, then they were good to go.
So around a year ago, Apple started a new division. A division focused on designing, engineering, and producing their own integrated circuits. They hired a lot of engineers from the microprocessor industries specializing in ARM based processing and the other area of focus was cell phone SoC (System on a Chip) integration and Software Radio design.
So the newest generation of iPhone software OS4 is actually specific for Software Radio technology using both GSM and CDMA frameworks. And if they come out with a new CDMA or dual framework phone this September as speculated, the new OS4 will be compatible.
And hopefully the new phone will be licensed and approved for multiple carriers.
My two cents....
Scott
