I'm pretty confident that you need to jailbreak and unlock the iPhone in order to use without data plan. But question bubbles whether it works with 3G or not. Google it and you sure will get an answer and get the firmware (One of my friends did the same: bought iPhone online and is using existing ATT simcard). ATT won't be able to find out that you are using iPhone. It takes just like 5 mins to jailbreak and unlock.