The cellular radio technology on recent iPads has been identical between the various models, but we've all had to buy an "AT&T iPad" or "Sprint iPad." There are ways to make a Sprint iPad work on T-Mobile by swapping SIM cards and so on, but some combinations never worked. The iPad Air 2 and iPad Mini 3 now ship with an Apple SIM. Per Apple's website, you'll be able to subscribe to service from multiple carriers with a single device and SIM. It looks like AT&T, T-Mobile, and Sprint all support it in the U.S. An unanswered question: What of Verizon? Presumably Verizon is continuing with its ESN whitelisting practice, which means you have to buy a "Verizon iPad" with a Verizon SIM pre-installed. Can you buy a Verizon iPad to preserve flexibility, then swap in an Apple SIM to gain access to the other three carriers?