Working on a review of the Garmin Nuvi 880 automotive GPS that will be published very soon, I paired the GPS with my iPhone 3G via Bluetooth, expecting there to be zero interop between the two.  To my surprise, quite the opposite was true.  Not only did hands-free calling work perfectly, but the audio came through the Garmin loud and clear.  I think I’ve found the solution to my dissatisfaction with the iPhone’s speakerphone, which, by itself, is hardly louder than its earpiece.  Look for the review in an upcoming issue of Macworld magazine.