I am in the process of doing this EXACT thing. My line (main) with 3g, is not eligable for the 3gS upgrade, my wife and her 2g however, is eligable.
A friend of my is a sales consultant at an ATT store, when i approached him with this idea, my initial thought was a sim swap right ? easy as pie right?
He informed me that on an iphone its not that simple. However he said it's just as easy for him to configure the swap on the ATT side as it is for me to swap sims.
So basically, my "wifes" new 3gs arrives friday the 19th...(shes not getting her hands on it for a split second), As soon as i get it i will activate through itunes as normal. Then once this is done all i have to do is call my friend at ATT and give him both IMEI numbers and tell him which one goes with which iphone. He said this is not illegal or un-ethical, simply put a loophole. He also informed me that this swap is pretty much the sole decretion of the store that you goto. So, my suggestion to all that dont have a friend at an ATT store, when you go in to get this done, be a polite and courteous to the consultant as possible, because technically they don't have to do this.
In closing, i really feel bad for all the wives of us avid iPhone users, their the ones really gettin screwed. LOL