Here's a short video in which Joshua Wright demonstrates how a Bluetooth headset can be hijacked, allowing audio to be captured or sent to the device:
Few users realize that Bluetooth headsets can be exploited granting a remote attacker the ability to record and inject audio through the headset while the device is not in an active call. SANS Institute author and senior instructor Joshua Wright demonstrates.
All that is necessary is knowing the device address, which can be easily sniffed, and the secret pin, which defaults to 0000. The headset audio is tapped while not in a call, so any room conversation the headset's mic can pick up can potentially be listened to remotely.