The big swapparoo
Thanks for all the fine info in this thread. I just want to make sure that I understand this correctly. We have a family plan and my wife and I both have iPhone 3G (I'd pluralize that, but it would look like 3Gs and be too confusing

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My wife's phone just died the wonderful WSoD (white screen of death) and we need to get her going again. Even though we've been ATT customers for years, she's not good enough to get an upgrade until sometime next year, so we're looking at $300-500 for a new phone (just plain ridiculous).
Being on the family plan, I proposed that we go to the ATT store on-line and add a line to our plan for our youngest and get that line a new iPhone 3GS.
When the new phone comes -
1) I put the SIM from my iPhone in the new 3GS and activate.
2) I put my wife's SIM in my old phone and activate (how does activation get triggered/happen? Do I have to wipe the phone first?)
3) I put the SIM that came in the new 3GS in the old RAZR we have lying around and it just works (right?) (but what about the data plan? - can cancel it?)
Thanks for any sage advice from those that have been though this,
Greg