We recently told you about Google opening (and quickly closing) the ability to port an existing phone number to your Google Voice account. Today Google announced they're re-opening porting to all existing users. New users will have to wait "a couple of weeks".
"Porting" is essentially transferring your existing number from one carrier to another, and in the U.S., Federal Law requires carriers allow you to more your number to another carrier if you tell them to. It's not free. Google is charging US$20 to port your number, and if you're still under contract with your current carrier they may charge you an early-termination fee which could be hundred of dollars.
There are some drawbacks. Google's number porting requires that you replace your current Google Voice number, rather than adding an additional number to your account. Porting will take a few days to complete, during which time service may (or may not) be interrupted.