I actually did the same exact thing today at the AT&T store.. very easy and helpful. They pre-ordered the new iPhone 3Gs as a new account. When the new phone arrives next Friday, I'll go into the store and they will merge it on to my existing AT&T account as an added line. I get the new iPhone for $199/$299 price and they'll put the sim from my current 3G into the new 3Gs and the sim for the new line of service into my current 3G for my partner to use. Having 2 iPhones that are basically a year apart in contract will theoretically allow me to get the newest iPhone each year, assuming Apple continues with releasing a new one each summer. By this time next year, my current 3G contract will be fulfilled and I'll be able to get a new iPhone for that line at the lowest price. 😎