The iPad will be with us for a long time. Features will change to keep up with technology. Voice features are not a big thing. If the carriers want to allow it, and it has a positive gain for Apple, all that is needed is for iPad owners to get a bluetooth headset. Even the original iPad had the capability. The issue is that with voice ability, the iPad will compete with cell phones and could cut into that market. And with the pay as you go data plan for iPads established, they could start a riot by insisting on contracts with iPads. Look at how much iPhones cost without contract, and how much money the carriers charge for voice and data services. The typical iPad with cell service is only $30 on a pay as you go service.
Voice usage is in minutes and data usage is in Mbps. Yet is it essentially the same general animal. The big difference is that as data speeds go up, the amount of data per second goes up. So if you were being charge by the minute of air time, you reduce the amount of time you might be on the network, but increase the amount of data. Voice is so low tech, that in terms of data transmitted, it is really small.