If your car has an audio jack (aux) but not bluetooth, I just ran across this Bluetooth audio receiver that is only $13. All other bluetooth devices are around two, three, or even four times the price. Its the size of a small ipod shuffle.
I just ordered one and plan to put in in the arm rest in my car where I have an audio connection. This way When ever i'm in my car I can play music via bluetooth to my car stereo without connecting any cables to my ipad. Will also be nice using the car stereo with the GPS navigation apps.
FM transmitters are not so great and tend to have static or cut out, with bluetooth It will be crisp audio and still remain wireless.
Cant wait to receive this device.
They have youtube videos posted of people using it with their phones and stereo systems, users say it works great with their iPhones, and its getting great reviews.
DealExtreme: $13.30 Bluetooth 2.0 A2DP AVRCP Stereo Music Receiver and Handsfree (Black)