It looks like the new iPhone 5 will have a serious roadblock when it comes to LTE, as it won't be able to place a call and handle data activity over the cellular network at the same time. This is an issue that plagued the non-LTE iPhone's, and is now plaguing the LTE-capable iPhone 5.
It is actually a design Apple have made in their design of the iPhone 5, so that customers could only do voice and data if the voice was going through the cellular network, and the data was being done over Wi-Fi.
Apple's iPhone 5, like many other smartphones, sport two antennas. The iPhone 5's antennas work together to improve the overall reception of a voice call in a single connection, where phones like Samsung's Galaxy S III throw the voice call on one antenna, and keep the data transferring through the other antenna.
This will only affect the CDMA-based carriers, but it is definitely something to think about if you're going to jump on Verizon, Sprint or any other CDMA carrier.