I am an iphone newbie. I am so confused by unlocked/jailbroken/etc. My family is die-hard tmobile fans. We love their rate plans and the customer service is fantastic. However, my dd really really wants an iphone and of course, its not offered by Tmobile. We would love to surprise her with one for Christmas. My inlaws have AT&T and have two phones eligible for upgrades with no desire to do so. They like their iphone 3Gs. Can I purchase their upgrade and then have it unlocked so we can use it on Tmobile? Will this work? Will it mess up the AT&T service for the 3Gs phone?