Google Voice now lets you port your existing phone number for $20
Number porting is available right now for all existing Google Voice users, and will become an option for new users over the next few weeks. To port a number to Google Voice, go to the Settings page and click on Change/Port next to your Google Voice number. The porting procedure is usually completed within 24 hours of your request.
After porting a mobile phone number to Google Voice, your existing mobile subscription will be automatically terminated, and you might incur an early termination fee from your carrier. Since Google Voice is not a mobile phone service provider, you will still need a mobile service plan, and another mobile phone number, if you'll want to use Google Voice with your mobile phone. Also be aware that you may experience issues with sending and receiving text messages for up to three business days after your number has been successfully ported.
If you already have a free Google Voice number on your account, the ported number will replace your old Google Voice number (once the porting is complete).
For more information on porting a phone number to Google Voice, see Google's FAQ on the matter. There's also a demo video waiting after the break.