Before you activate that new iPad Air 2 or iPad Mini 3, you should consider which carrier to use it with. Both of the new iPads come with a special Apple SIM card that allows users US and UK users to switch between wireless carriers on the fly without removing the SIM card.
'The new Apple SIM is preinstalled on iPad Air 2 with Wi-Fi + Cellular models. The Apple SIM gives you the flexibility to choose from a variety of short-term plans from select carriers in the U.S. and UK right on your iPad. So whenever you need it, you can choose the plan that works best for you — with no long-term commitments. And when you travel, you may also be able to choose a data plan from a local carrier for the duration of your trip.'
Participating carriers in the US are listed as AT&T, Sprint, and T-Mobile. This is possible because there is a single cellular model of the new iPads that works on all carriers. Because of this there is no need to select a carrier when buying an iPad Air 2 or Mini 3 on Apple's website. To use one of these iPads on Verizon, you'll have to acquire a SIM from Verizon (which they may charge a few dollars for). I'd hope that if you buy an iPad directly from Verizon it will come with a SIM included, though.
However, MacRumors reported today that if the Apple SIM is activated on AT&T that SIM will be locked to AT&T. The iPad itself will still be unlocked and usable on all carriers, but a new Apple SIM or carrier-provided SIM will be required to make that possible.
AT&T gives away SIM cards for free, even if you aren't a current customer. If AT&T is your pick, you may want to get a SIM from your local AT&T store to active the iPad on. That way, you'll still have the Apple SIM if you later decide to switch to T-Mobile (who charges for SIMs) or Sprint, or if you'll be traveling abroad and want to easily switch between carriers. I spoke to Apple Support today and they said replacement Apple SIMs would be available at Apple retail stores, but pricing info was unavailable.
When making your choice you may want to check out our comparison of tablet data plans. There isn't much differentiation between the carriers other than coverage. We recommend AT&T as our overall top pick due to its coverage, value, and unique flexible options. Verizon has the best overall LTE coverage, though, and T-Mobile gives away a little bit of free data every month.