Is there anything I need to worry about with an iPhone on At&t prepaid?

I am going to be buying my friends JesusPhone soon. The original 16gb. He has it jailbroken. I am going to buy it from him for $200. There is no way I can justify paying $90 a month for an iPhone plan, especially considering that I never talk on the phone. So prepaid seems just right. I remember that there was a scare back in August I think with the 3.0 software coming out that At&t said that they will find you if you are using prepaid and they will make you sign up for the regular plan. But my friends iPhone is jailbroken. Basically what I am wondering what I must be weary of with having a prepaid iPhone, thanks!