Stereo Bluetooth transmission/reception requires the A2DP Bluetooth protocol, which is only in the 3GS iPhone; the Original “OG” iPhone and the 3G are monaural Bluetooth only, which works with car audio systems set up for hands-free telephone functions. Even though the 3GS now has A2DP, most car systems are NOT A2DP, to the best of my knowledge. I have not heard of any car audio adapters or aftermarket systems that have this feature.
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@Deepness: Does your BMW cable charge the 3GS? My MINI Cooper S (which is engineered by BMW) came with a cable that ran the controls off the steering column that also charged my OG iPhone, but I get a “Charging is not supported with this accessory” message with my 3GS… so I was just curious if yours works or if you need a new cable. I was told by my MINI/BMW Dealer that a compatible cable is not yet available, and the converter that Apple sold me was only for the 3G, not for the 3GS.