When Apple released iPhone firmware 1.1.1 they may have inadvertently enabled a new, undocumented feature.
According to AppleInsider, iPhone 1.1.1 allows you to stream all audio, including iPod songs, audio from movies and TV shows and even keyboard taps and system sounds to a Bluetooth headset. Previously, only phone calls and voicemail (when enabled) were streamed to Bluetooth headsets.
Apple appears to have enabled the A2DP (Advanced Audio Distribution Profile) Bluetooth profile in the 1.1.1 firmware update. The only problem is that it isn't implemented properly. While streaming audio to a Bluetooth headset iPhone simultaneously outputs the same audio through its built-in speaker, which makes the feature much less useful. Plugging in headphones only serves to cut off audio to the wireless headset.
The question is what Apple will do to "fix" the bug. Will they silence the speaker when using A2DP or will they remove A2DP streaming for audio other than voicemail? I hope that it's the former.
Update: I removed a portion of this post that referenced streaming voicemail messages to a Bluetooth headset which erroneously stated that this functionality was added in firmware 1.1.1. The PowerPage's Rob Parker informed me that this has been possible (via the "Audio" button in voicemail) since the beginning.