I have an AT&T/Cingular phone now, and was wondering how it would behave (a general cell phone question, perhaps) if I was to activate a new iPhone, with my current phone number, with the iPhone's SIM card- if I would still be able to turn on my existing phone and get numbers out of the address book, and use the other phone features other than actually making calls. Without a SIM card at all I know that's not possible, but my impression is that you use the card that comes with the iPhone, so my old phone would still have one.
Yes, ideally I'd have my phone now sync'd with my desktop, but last time I checked, at least, it wasn't supported by iSync, and I don't want to lose my data.