Short answer: try reseting your router.
Long answer: ISPs normally don't want you to be able to screw with your IP address for a number of reasons, and they tend to change it whenever they like (unless you have a special business type class connection). So, there is a chance, albeit a small one, that your old IP address will crop back up. Also, your hostname will remain the same, so if someone wants to trace you, like the RIAA, all they need is your hostname (and a subpoena).
Oh, and thread moved.