Even with the few people that have the service and it’s still limited to invite only users, Google Voice officially started just 3 weeks ago and is now available for BlackBerry and Android Phones, as referenced from Google Voice Blog. It is expected to be available soon to iPhone, per NY Times, and will be expanded to other compatible phones as well. Excerpt from the Google Voice Blog -
Thanks to some great work by the Google Mobile team, we now have a Google Voice mobile app for Blackberry and Android-powered phones. The mobile application makes using Google Voice with your cell phone simpler by letting you place calls and send SMS messages directly from your Google Voice number. You can call from your phone's address book or enter in numbers directly, and your outbound calls will display your Google Voice number as the caller ID. You can also listen to your voicemail and read transcriptions of your messages.
Google Voice, formerly known as GrandCentral, is a service that gives you one number for all your phones, voicemail that is easy as email, and many enhanced calling features like call blocking and screening, voicemail transcripts, call conferencing, international calls, and more.