AT&T, Sprint, T-Mobile, and EE (in the UK) were signed on as the first carriers to support the Apple SIM - with more promised in the future.
But now that the new iPads have shipped, it turns out that AT&T is being a bit dastardly about how they are providing "support".
It turns out, once you pick AT&T - the Apple SIM (just the SIM, not the iPad) becomes locked to AT&T and will never allow you to select one of the other carriers. In other words - it transforms from a flexible multi-carrier SIM to a single-carrier AT&T-only SIM.
Apple posted a support document confirming this behavior:
Using Apple SIM, you can choose from different cellular carriers and their various programs. The data plans vary by carrier. For instance, in the United States, you can choose a domestic plan from either Sprint or T-Mobile and also pick an alternate plan from the other carrier as needed. When you choose AT&T on iPad Air 2 and iPad mini 3, AT&T dedicates Apple SIM to their network only.
If your Apple SIM becomes dedicated to a specific network and you want to choose from other carrier programs, you can purchase a new Apple SIM from an Apple Retail store.
If you choose Sprint or T-Mobile, the Apple SIM remains unlocked and will continue to allow you to easily switch carriers. Even to later elect a one-way trip to AT&T.
Of course - all iPad's are completely hardware unlocked and can be used on any carrier, and even if the Apple SIM ends up locked to AT&T you can always use a SIM card directly from any other carrier to get online. Just use a paperclip to pop out the old SIM and swap in a different one, the same way it has always worked in the past.
Verizon, who is not signed up for the Apple SIM program, is providing iPad support this way.
The Apple SIM is a great idea and opens up a lot of doors for both increased customer flexibility and competition. I guess it is no wonder that the biggest carriers are dragging their feet.
T-Mobile on the other hand is actively bragging about how they enable customers to "stay flexible", and as a launch promotion they are offering 5GB of pre-paid data on the iPad that does not expire for 5 months - all for just $10. This is an awesome $2/GB price.
Maybe that sort of competitive offer is exactly what AT&T is afraid of...