I suspect when Apple is ready to have the iPhone on LTE, it will be on both carriers at the same time. It does not make sense to make two different iPhones for different carriers. In fact, the Verizon iPhone 4 has Qualcomm's CDMA/GSM chipset in it. Apple did not want to reinvent the wheel on the iPhone 4 hardware design, otherwise, the Verizon iPhone 4 could have been a "world phone." The Qualcomm's CDMA/GSM chipset is the same as the one used in the Droid 2 Global and Droid Pro Global.

Best thing for Apple to do is to use a chipset that supports CDMA/GSM/LTE and make just one phone which will work on multiple carriers.

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