using sim from another att phone in my iphone

You are going to be using data and charged for it anytime you use visual voicemail, Google Maps, Weather, stocks, etc.

You may be able to get by with doing this for a period of time, but sooner or later AT&T will pick up on this in their system since the chances of you never using any data via AT&T's network are slim to none - ether on purpose or by accident.

But this kind of goes with my other thread... clearly you can swap sim cards/numbers between an iPhone?

And then I could just re-add the data package onto the other number unless its covred under my family plan?

No you cannot clearly do this.

I still have a first generation iPhone that I used for over a year before purchasing an iPhone 3G. I cannot and to repeat, I cannot move the SIM card from my iPhone 3G to my first generation iPhone, turn it on and place/receive calls. I can move the SIM card from my iPhone 3G to an AT&T carrier locked phone that I was using prior to purchasing my first generation iPhone, turn it on and have service, but the same does not apply to the iPhone.

Why the OP is able to do this without going thru the iTunes activation process is a mystery, but it is the exception rather than the rule.

If I was going to guess, both phones have the same phone number rather than different numbers.