IPhone 3G S data plan?

Hey, I have an important question and I was wonder if someone could answer it for me. Anyways I am wanting to get the new iPhone 3G S and I was planning on getting it but I heard that in order to be able to have the iPhone you have to have a whole new separate data plan for it by itself. Is that true? I am hoping not but oh well if it is but also I wanted to say that I am upgrading from my BlackJack II to the iPhone and I was wondering since I am already on a 5 line family data plan can I just use my existing family plan I have now instead of paying an extra $30/month. I just don't see how that can make sense. Anyways anyone that can answer this please help me.