I haven't specifically found this information on here yet so I figure I would share it to make it easier for anyone looking to use a Garmin GPS & pair it with a bluetooth headset.
According to Garmin, the only Garmin Devices that support pairing with a Bluetooth headset are the nuvi 765, 775 , 785, StreetPilot 2820, zumo 220*, 550, 660 and 665.
When a headset is paired with these devices the listener will be able to hear phone calls, navigation prompts, or MP3 music (if the device has a MP3/Media Player).
All other Garmin Bluetooth-enabled devices are only capable of pairing with a Bluetooth cell phone.
The exception is the zumo 220. It will only pair to a Bluetooth headset or helmet and does not have the ability to pair to a Bluetooth-enabled phone.