Hey is this completely allowed? I was doing this today. I was taking my sim card from my iPhone in and out of a N75 like 2 times total, and then I got a weird text from AT&T. They don't charge you, when you do that, do they?
Nope, no charge. They are able to see the phone you are on, so if you call in for maintenace for some reason, make sure you have the sim in the phone they have you registered to. Although I have heard that they still know if you have been using a different phone up until that point.