Google announced Voice mobile app for Blackberry and Android. According to their official blog:

From the mobile app, you can:

• Access your voicemail: read message transcripts, follow along with "karaoke-style" playback of messages, read SMS messages sent to your Google Voice number (even if your phone doesn't receive SMS messages) and access your call history
• Place calls that display your Google Voice number from your address book, the app dialer (Blackberry) or the native dialer (Android)
• Send SMS messages that display your Google Voice number
• Place international calls at low rates

See how it works in the following video: