A co-worker uses this. Pretty slick. He was showing me how this 'Google phone number' is tied to his cell number. Anyone that calls his Google Number, the call gets forwarded to his cell phone. If he doesn't answer the phone, a message he created under his Google Phone account plays and allows someone to leave a voice message. If a message is left, he receives a text message on his phone with the text of the message, and he also receives an email in his GMail account with a MP3 of the message that he can listen to. There's a CallBack button that he can call the number back that left the message. The Caller ID is his Google Phone Number, not his cell phone number.
Like Nick stated, someone with an existing account has to 'invite' you to join. Each person that has a Google Voice account has a limited number of 'invites' they can use. Don't think Google is handing out invites any longer. Think they only offered them to people that initially signed up for Gmail and Google accounts some time ago.