Google is testing a feature that could allow users to port their existing number to Google Voice. Check under phone setting to see if "change / port" is now an option. After checking to make sure Google is able to port a given mobile number, a $20 payment via Google Checkout is all that's required.
Google notify this feature is still being tested and may not be available to everyone. "We're continually testing new features to enhance the user experience. For a limited amount of time, we're making the Google Voice number porting process available to users. We don't have any additional details to share at this time, but plan to offer this feature to all users in the near future."