It has already been established that bluetooth headsets can provide passable audio quality and can be really handy. By default, if you have a bluetooth headset connected to an android phone, it will automatically open the "Voice Dialer" activity. This is handy if all you want to do is call someone, however it could do so much more by simply using the Google Now instead. Google Now has all the functionality of the voice dialer, with about a million additional things.
Until this default behavior is addressed, which I'm sure it will be at some point in the near future, you can get your bluetooth headphones/speaker to play nice with Google Now. Simply try out Bluetooth Launch (free), written by a dev who was obviously after this functionality, but wrote the app in such a way that you could open whatever activity you wanted.
How to set it up
All you need to do is download Bluetooth Launch to your device, open it and assign it to
Google Search > com.google.android.googlequicksearchbox.VoiceSearchActivity
and finally connect your bluetooth headset to the device. Hit the button on your bluetooth headset and select "Bluetooth Launch" as the default and you should be good to go. You will also make sure Google Now is using your bluetooth headset as an input, so make sure to check the box in
Google Now > Settings > Voice > Bluetooth headset (checked)
To get rid of it or start over, you can simply uninstall the app or clear the defaults in the Bluetooth Launch app info page in settings.
Due to all of the differing bluetooth device setups, which are notoriously finicky, your phone/bluetooth headset may not be compatible or work right away. I tested and was able to get this to work on my Nexus 5 & LG HBS-700's but not my Nexus 7 with the very same headset. So, as with most things you find around these parts, your mileage may vary.