My phone number is the killer feature of my phone. I've had the same mobile number for about 5 years, and I'm not letting go. The phone companies know that your phone number is a valuable asset as well and "wireless carriers are not required to provide telephone number portability at this time." This is a competitive advantage leveraged by one of the most basic usability rules that familiarity makes things easier. Changing your number doesn't breed familiarity, and prevents me from adopting competitive technologies.
And don't let the wireless carriers fool you, number portability between GSM and CDMA networks has been available in Australia since 25 September 2001.