Prior to the iPhone software version 1.1.4, you could listen to the iPod audio over a Bluetooth headset if you did the trick of switching to the voicemail screen and selecting the headset for audio. This still works, but you can't leave this screen and you can't sleep the phone, or the headset audio is disabled.
What I found was that you can get the old functionality back by doing the following:
If you leave the Bluetooth audio selected in the voicemail screen, you won't have to select that every time. The headset audio will activate as soon as you go into the voicemail screen. It may sound like a bother, but it's not too bad once your fingers get used to the sequence. Obviously, you have to have your home button double-tap setting set to go to the phone favorites, and enable the iPod controlls.
Of course, you wouldn't and can't listen this way for a long time, but this is useful for listening to podcasts in the car. I wear the headset anyway, and my drive is only 15-20 minutes each way, so one charge gets me through the day. Also I don't need my podcasts in stereo.
It seems the "bug" that allows this is the phone screen turns off headset audio if you leave the voicemail screen. However, when returning to the phone app via the home button shortcut, it re-enables the audio because it thinks it is going back to the voicemail screen, but gets redirected to the speed dial screen in a way that bypasses turning off the headset audio.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20080612195428571