My experience with moving an upgrade from the line ordered under to a differnt line :
Phone bought from Apple
Having previously read (before finding this thread) that I should go to an AT&T store, I was there as they opened this morning. I brought the unopened iPhone 5 with me, acquired using my sister-in-law's upgrade (who is 800+ miles away). I stressed to the agent that I did NOT want to suffer the wrath of the sister-in-law

by deactivating her line, so please make sure we do this right!
I didn't pay close attention to what she did, but whatever it was was without even opening the phone. She then said it was all ready, just plug into iTunes and everything was set. At that point, I realized my current line was deactivated, so she said she could activate the new phone.
She powered it up and got to an activation screen stating that:
Activation
XXX-XXX-XXXX
The phone number should match the one
provided when you purchased you
iPhone. If you want to use a different
number, contact AT&T after activation
(with XXX-XXX-XXXX being the wrong number)
Long story short, at this point, after a 45 minute call to Apple and to AT&T, I know that the IMEI and ICCID of the nano sim is correctly attached to my phone number. (And a very important point, I have been assured that my sister in law's phone service is active and untouched

). However, my iPhone 5 still shows as being attached to my sister-in-law's number when I go through the activation process, both through iTunes and through just turning the phone on.
So I'm working with an Apple senior rep who has escalated to an "engineer" and I'm waiting for callback.