Is it possible for me to walk into an Apple store in the US and buy an unlocked unsubsidized AT&T version iPhone 5 without an AT&T account? Based on what I'm reading, it will be locked until I restore it in iTunes, then I can use a SIM from another carrier. The question is...will they require me to sign up for AT&T service?
Just to contribute to the simultaneous voice/data question- LTE only supports simultaneous voice/data if the voice channel is VoLTE...and carriers are not quite there yet. All iPhone 5 versions only use LTE for data. If you are on a traditional GSM carrier, like AT&T, the device drops back to HSPA+ for simultaneous voice/data. If on a legacy CDMA carrier, like Verizon/Sprint, the device drops back to CDMA and won't do simultaneous voice data. (Apple did not want to add another radio just for CDMA carriers to this...like you can with CDMA and WiFi) That being said, the LTE handsets on the market today are just temporary bridge devices until the carriers move to VoLTE. I expect that the CDMA carriers will be the first to move to VoLTE, but who knows if Apple will support it in the iPhone 6. I bet Samsung will support it first as a differentiator. VoLTE tech is here today, but the carriers need to settle how to commercialize something that is more like Skype and less like a switched voice circuit. Just Google for "VoLTE" to learn more.
It will be a great day when we can buy a handset and use it on our carrier of choice based on price, network performance and customer service (like home ISPs) rather than bringing stuck with a carrier because our handset is locked to that carrier's network.