Google Visual Voicemail is a semi-offline client for Google voice. You can view and play your voice messages as well as mark them read/unread and delete them. When updating, message text is downloaded to a local cache that can be viewed at any time. Audio is downloaded the first time you click play and is stored for future use. You must update manually by going to menu->update. Automatic voicemail checking is being worked on, keep an eye on the NEW section.

http://forum.xda-developers.com/showthread.php?t=671086