I've got AT&T. I had an iPhone 4. I yanked the Sim Card out, made an adapter out of a thin piece of plastic, and slapped it right into my Note. Total elapsed time: about five minutes. It worked perfectly, and was running HSPA+ speeds.
Then today, I decided to sell my iPhone 4, so I wanted to put the original micro sim back in it (short version: it won't restore to factory settings without a sim card in it). So I borrowed a friend's old AT&T iPhone 3G (regular sized sim) and went into the local AT&T store and told them I wanted to switch to using the old phone. They activated a new sim card for me free of charge. I walked out of the store, popped that newly activated sim into my Note, rebooted it, and I was back up and running in about 30 seconds.
Technically, I should tell AT&T I'm using an android device now, but I don't see the point. Everything works.