It's not a real SIM card. They talked about it in the keynote; it's a built-in electronic SIM chip or something.
So in other words no way to get this directly rather than from a carrier and no way to switch carriers unless they explicitly support it.
Sounds great.
It uses your phone's carrier.
An "electronic SIM card inside". I don't think this means there's a physical SIM. Dunno how that's going to affect compatibility with carriers, which is OP's question.
Is this the Apple SIM[1] that lets the iPad choose a carrier as a configuration setting?