Preview at WWDC likely in June, followed by September launch.
Verizon iPhone 5 Will Not Offer Simultaneous Voice and Data [Update: It Was A Hardware Decision]
The AT&T iPhone has supported simultaneous talk and data since the iPhone 3G was released because of the GSM network it uses.
Verizon gave this statement to MacRumors:
iPhone 5 was designed to allow simultaneous voice calling on the Verizon Wireless network while browsing the Internet over WiFi. This is no different from the current iPhone 4S.Sprint has not yet confirmed whether its iPhone 5 will support simultaneous voice and data, but it uses the same physical hardware as the Verizon iPhone. The Verge assumes that it will be behave the same way.
Update: According to The New York Times, it was Apple's choice to prevent Sprint and Verizon phones from using both LTE data and voice on simultaneously. Because the LTE network only supports data and not voice, Apple would have to add a third antenna to the iPhone 5 to allow both LTE data and CDMA voice together.
An Apple spokeswoman told The Times, "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."
From the Times:
So why does Verizon’s Samsung Galaxy S III, a 4G LTE phone, juggle calls and data? Samsung added an extra antenna so that it pulls data from the 4G LTE network at the same time that it’s using another antenna to do voice, said Anand Shimpi, editor in chief of AnandTech.
Then why didn’t Apple add another antenna? It actually already has two antennas in an effort to improve reception, and it would have had to add a third antenna just for Verizon and Sprint phones to give them simultaneous data and calls, Mr. Shimpi explained. Leaving that third antenna out allows Apple to simplify its manufacturing process of the iPhone for multiple carriers. Plus, in the next two years the 4G LTE network is supposed to evolve to support voice calls, which would render another antenna unnecessary later.