I also have my own unlocked iPhone 3G (not purchased through at&t). You can do whatever you'd like in terms of service. As mentioned in previous threads, you don't need to sign a contract. Also, you can choose one of at&t (or T-mobile) pre-paid services, which are really inexpensive. It all depends on your usage pattern. Whatever you do with your own iPhone 3G will have absolutely no impact on your potential future purchase of an iPhone 4.