I am probably late, but I just found out today when i went into my Google Voice settings that you can now forward your voicemail from your cell number to your Google Voice number. This is awesome if you want to keep all of your voicemails in one place and get them transcribed (although some transcriptions are really, really funny!)
I have an iPhone so I am not really too interesting in doing this since I have Visual Voicemail anyway. But for those without Visual VM, it can be great. To get started click Settings–>Phones. For more information about how this will affect your calls and VM check out the Help Section.