Two ways to do this:
Old iPhone, activate it via iTunes and insure you fail the credit check. You will be offered the GoPhone plans.
or
Buy a Cheap GoPhone and activate it. Take any iPhone including 3G and make sure it is activated. Place the GoPhone sim in the iPhone and now you have a GoPhone iPhone. If you have the 3G iPhone, the 3G service will work perfectly since all that is required to get 3G service is a 3G phone.