Those of you pumped up about the iPhone 5 bringing in architecture that allows simultaneous voice and data on a single radio, take heed: it's been confirmed that this ability only works on GSM-based LTE (and 3G) networks. If you're planning on working with a network that uses CDMA for voice and LTE for data, you'll be out of luck - they'll both work, just not at the same time. The key here may be in the official Apple statement (as seen below) which includes a "not yet" which may keep some doors open.
If you're working with AT&T with the iPhone 5, you'll be on GSM, and will be able to work with simultaneous voice and data right out of the gate. If you're on any other carrier here in the USA, you'll be working with CDMA, and will not yet have that functionality. Have a peek at the official statement from Apple on this situation.
"iPhone 5 supports simultaneous voice and data on GSM-based 3G and LTE networks. It is not yet possible to do simultaneous voice and data on networks that use CDMA for voice and LTE for data in a single radio design." - Apple
Again keep your eyes on that "not yet possible" and decide for yourself whether or not that means that the operating system will be able to be tweaked to a degree in the future to allow both voice and data at the same time over LTE. For those of you that do plan on purchasing an iPhone 5 and have not yet chosen a carrier, take a peek at the timeline below with a collection of iPhone 5 news bits and updates to help you see.