Did you sign up for an iPhone family plan?
Ok I found my brochure that the AT&T store gave me at the launch day. If you signed up for a whole new family plan with AT&T and signed up for the iPhone family talk plan, then no everyone on the plan gets unlimited internet. Say you signed up for the $109.99 (or whatever) then you would not need to pay $20 per line for data. But if you are an existing AT&T customer, and want to add the iPhone onto your current voice plan, then yes you would need to add $20 per line.