So since I'll be moving out of the US in less than a year, I don't want the 2 year contract and I want a phone I can bring back to Canada, so I'm opting to buy the unlocked iPhone 4S from the Apple Store (available in November). Question is, will I be able to use my sim card from my 3GS or will AT&T give me a new one with no problem?
If you bring your own phone to AT&T, they don't care. Since they aren't subsidizing a phone you got on you own, all you pay is for the service. If you already have service with them, they don't care if you swap phones. You're still paying them.