From YAHOO:
Beginning on November 24, 2003, many wireless subscribers will be able to keep their existing mobile telephone number when they switch wireless service providers. This ability, known as telephone number portability or local number portability (LNP), is being implemented by the Federal Communications Commission (FCC) under the direction of Congress.
The goal of LNP is to promote competition and innovation in the marketplace and to offer consumers more choice in phone products and services, including the ability to retain your phone number without a loss in quality of service if you decide to change carriers.