I don't know what's preventing me from understanding this, I've just been so wrapped up in the Pre launch and WWDC I can't think correctly! When the 3GS comes, won't it make me activate it on the line that I ordered it through first? I don't see how it would let me activate it for a line that isn't eligible for an upgrade at the beginning.
Assuming I'm right, I'd need to activate the phone on the other line, sync it with that person's iTunes, then pop my SIM in it after I just set it up with the other SIM?
Yes, thats basically what I said.
You will have to follow through with however you're suppose to activate the 3Gs, and it will have the other person's phone number and everything at first.
THen you will need to take that sim out, once it's activated, and give that sim back to the person who will need it in their old phone. If that old phone is a first generation iPhone, they will have to go through the AT&T activation steps again on their phone via itunes, selecting replace current phone, etc, but if htey have a non-iPhone or iPhone 3G, when they plug in the sim from the 3Gs, they will be good to go.
You will also be good to go, when you plug your current sim, into the 3Gs that you at first activated with the card it came with, again, before passing the card the 3Gs comes with back on to the other person.
However, one important thing, you will have to get AT&T to remove the $30 data package from the other person's line, tell att the iphone broke or something, and that line is back to an old phone. Or, if it is first generation iphone and you plug the 3Gs sim into it, when they do the AT&T steps through itunes, again something that only happens with first gen iphone, they can get back on the $20 data plan that way.