It seems for years there have been many apparently damning articles (or at least the headline only) attempting to discredit the security of Bluetooth. The latest of such had recently been featured on digg. For those of you who did not watch the video (or simply read the headline), the guy pairs to a device that is inherently flawed. It is not bluetooth that is insecure, it is the device which apparently uses a guessable PIN and doesn't require a pairing sequence. The 'hacker' does not listen in on a phone conversation- he pairs to the device and listens to (and 'injects') arbitrary audio. This is not a hack, it is a remote pairing to a device that apparently breaks all published bluetooth spec's.
For those who are still worried about bluetooth security or eavesdropping, the only way for this to happen is:
(a) some severely under developed device such as the headset mentioned in this article,
(b) someone eavesdrops during a pairing procedure, which is most likely performed within the owners own home- which the only thing that would be discovered would be the pin- not the device info. Once the device had been paired, there would be no way to 'unpair it' which would present the risk of anyone with a bluetooth host to capture any device at random.
Why stop at headsets? Phones and even PC's have bluetooth functions. BlueBug is a blog which features articles about 'bugs' in bluetooth devices, which are actually just implementation flaws on the part of the company that implemented bluetooth in their device- not in bluetooth itself. I look at these problems and have come up with a good analogy: it is like having a sports car with a key that can never be copied and without it the car can never be operated, but the manufacturer decided to glue it in the ignition.