I recently found this news article from April. It is a few months old, but is interesting.
How Verizon Wireless May Kill Hope for LTE Interoperability - Businessweek
Basically, AT&T and Verizon are using different portions of the 700 MHz band for their LTE networks. It is even worse with Verizon reversing its duplex in its portion of the 700 MHz band.
In addition, Verizon is planning to abandon 700 MHz band and use its recently acquired 1700/2100 MHz bands for LTE.
This will make it more expensive and impractical to make global phones like the HTC rezound which will work on any cell carrier.
This also implies that in a few years all Verizon phones may become obsolete because if their inability to operate LTE on the 1700/2100 MHz bands.
Our HTC rezound might only work on gsm networks in a few years time?