I have a similar situation. When I got the 3GS I gave my wife my original iPhone after unlocking and jailbreaking it, then put her SIM card in from the free Sony they gave her (on our family plan).
Under my AT&T account her iPhone still shows up as the Sony phone, but shows a picture of the iPhone. There's no data plan on it and I don't see that we're being charged for data. Her iPhone can't get any data unless it's on Wifi.
I'm wondering now, since I have a new iPhone 4 on the way and she dropped her iPhone in the sink 🙂() if I can just switch SIM cards in my 3GS and pass it on to her, or if I need to unlock it. I don't remember exactly why I unlocked her phone in the first place.
Should I wait for an unlock? Is there a definitive answer on this type of situation?