Got the latest firmware update and set my settings as directed in many forum posts, and this still isn't quite right.
I found that the VM indicator turn OFF more dependably if I check the VM using the dial-in interface, and if I call to do that using the SP1 line which is aassociated with that Google Voice account.
However, I STILL have the problem of new voicemails not turning the indicator ON for a LONG time after the message is left. This problem seems worst if the message was left without ringing the Obi (for example if the caller is set up in Google Contacts as someone to go straight to voicemail.)