Saw some references to Google releasing a native Blackberry app this morning to support Google Voice.
You can download the app OTA from m.google.com/voice.

You will still need a Google Voice account.

The app will ask you which device this is. The list shows the devices/numbers you setup online in Google Voice. You can choose to listen to voicemails over the handset or via the speaker.

The Inbox shows the messages you have received.

You can drill down to see the transcription and also play the original message after it has been downloaded.

An additional option, is to dial out through the application, routing your call through Google Voice. Your Google Voice number will show up on caller id.