Be careful about berating other users without knowing what they are experiencing. I AM familiar with my phone, my bluetooth headset IS paired properly and the "AUDIO" button you refer to is marked only "SPEAKER" regardless of the status of my bluetooth headset.
According to this article, the change was intentional (boo!):
http://www.macworld.com/article/134451/2008/07/iphone3greviewnotes.html
Quote:
Borchers confirmed that the iPhone’s Bluetooth software has been modified to prevent you from listening to your voicemails via Bluetooth headset. What he told me was that Apple wasn’t really satisfied with the user experience of having to go to Voicemail and then tap on a button to set the output source to the Bluetooth headset—and was especially worried that it might be dangerous while driving. However, don’t despair—he also said that Apple is working on a replacement for this feature that will allow Bluetooth-headset wearers to listen to their voicemail again in the near future.
===
The iPhone users guide, on the other hand, says the button should be marked audio when a headset is connected. Thus far my iPhone 3G definitely does not behave that way, but others report that their phones do. So there is a contradiction not only from Apple, but also in end user experiences.
I would really like to get it working agin, as I now have no way to listen to voice mail in the car.