To tinker with this, you of course need to have a Bluetooth headset paired with your iPhone. I personally have an older HS-850, one of the first Motorola models with a microphone that flipped open to toggle the headset on and off. After you've switched your headset on and it's shaken hands with your iPhone, go into the Visual Voicemail screen and tap the Audio button in the upper right (which is otherwise a Speaker button when your headset isn't on and communicating with your phone). You'll receive an audio selection dialog much like the in-call dialog that allows you to chose where to send the iPhone's audio. As you might guess, select the Headset option, then switch over to the iPod and begin playing music or a video. This, unfortunately, is why I had to include 'kinda' in this post's headline - while the iPhone sends audio to both Earle's Motorola H700 headset and my HS-850, it still plays audio though the iPhone's speakers as well. It's a bizarre quirk I can't seem to figure out how to stop, which makes me think this is either an unfinished feature or, perhaps more likely, simply a side effect of some of the iPhone's functionality.
Whether you can use this trick to get the iPhone to send stereo audio to a true set of Bluetooth headphones I couldn't tell you, because I own no such headphones. If you readers tinker with this more and help uncover more of what's going on here, please sound off in the comments.