BusinessWeek found some information about Google Voice in a response to an inquiry from the Federal Communications Commission, an independent agency of the United States government:

"Google Voice, which provides people with a single phone number that can be used to reach them on their work, home, or cell phones, has 1.419 million users, according to the letter. Of those, 570,000 use it seven days a week, Google says."

The service is still limited to the US and it's available by invitation. Google Voice users can invite other users from the web interface.

Google now offers an additional version of the service for those who want to use their existing phone numbers. This version is more limited and it only offers voicemail-related features and international calling.

"Google Voice charges only for outbound calls to international locations; at present, fewer than 4% of all Google Voice users place outbound international calls," explains Google.

Google emphasizes that Google Voice is not a phone service, it's a Web-based software application. "Google Voice works with mobile phones, desk phones, work phones, and VoIP lines. There's nothing to download, upload, or install, and you don't have to make or take calls using a computer."

Google confirms the acquisition of Gizmo5, a proprietary SIP soft phone. "The Gizmo5 network uses open standards for call management, Session Initiation Protocol (SIP) and Extensible Messaging and Presence Protocol (XMPP). However, the Gizmo5 client application is proprietary software and uses several proprietary codecs, including GIPS, iSAC, the wideband adaptive codec made popular by Skype," explains Wikipedia. Gizmo5 is "the only SIP service without PSTN-based US phone numbers that may be used with Google Voice."

"While we don't have any specific features to announce right now, Gizmo5's engineers will be joining the Google Voice team to continue improving the Google Voice and Gizmo5 experience. Current Gizmo5 users will still be able to use the service, though we will be suspending new signups for the time being, and existing users will no longer be able to sign up for a call-in number," mentions Google.

I wouldn't be surprised to see Gizmo5 morphing into a new version of Google Talk, with built-in support for Google Voice.