The GrandCentral service features a central voicemail inbox that can be accessed online, a single phone number that will reach your home, mobile and work phone, and a way to listen in on a caller as he leaves a message on your voicemail. All of these applications will be available on Google Voice. You can check out the features page for more details. In the meantime, members of GrandCentral will receive information in their mailbox on how to join Google Voice. If you are not a member of GrandCentral, don't worry, the service will soon be made available to everyone. You can be alerted by email when membership opens up to everyone by going to the source link and sending an email.
source: Official Google Blog