As a matter of fact I have an auction for an Unlocked Sony Ericsson T616 (originally with ATT) and I have used it with many carriers and it works just fine. There are no repurcussions and this way you do not have to sign up for a new 2 year contract. E Bay is a great source for used and new GSM cell phones both locked and unlocked. My auction ends at around 6.40 PM EST, if you search I am sure you will see mine and many more similar ones.
Sorry if this topic has been beaten to death. I've read through a few of the related discussions, but my situation is kind of unique.
Yesturday, i went to a Cingular Wireless distribution stand to just upgrade my crappy motorola AT&T phone to a new Samsung camera phone. I found out that i was under a Corporate Discount program, since my dad works at Fidelity Investments. Also i'm in a family plan which means everyone in my family has the same phone, # of minutes, etc.. Apparently the salesman was unable to "touch" my account, period.
I was just wondering if i could purchase an unlocked AT&T phone off of ebay and just replace it with my old phones SIM card and be GOOD TO GO without any repurcussions.
Thanks in advance.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic