I've using my Treo via a FM transmitter for months without major issues. But instead of opening my FM transmitter, I made my own adapter with an old 2.5mm headset, 2 * 500 ohm resistors, and a 3.5 mm stereo jack. I keep my FM transmitter on by listening to pockettunes. The music will pause when I am on a call (you will need MagicButton if you want to make a call and want auto resume after the call). I fact, TomTom voice will speak over the music normally, then resume after the call as well. The only small issue is that the car charger seem to add a bit of noise sometimes ( I think it is related to cell tower switching or data access). By the way, I disabled the headset boost in volumecare so the music volume will come back unchanged (volumecare seems to forget to set the volume back to the original level after a call).