With Google Voice mobile app for Blackberry and Android, you can make calls and send SMS messages with your Google Voice number directly from your mobile phone, as opposed to using Google Voice. The app is fully integrated with each phone’s contacts, so you can call via Google Voice straight from your address book. You can download from Google Voice mobile site, or search for "Google Voice" in Android Market. For those, who don't have Blackberry or Android device, can access, mobile version of Google Voice site from mobile browser. You'll need a Google Voice account to use the mobile application.
More info: Mobile blog