check out Griffin's products, I have one that works well (don't remember the model, sorry). It has a transmitter that plugs into the headphone jack, with a car lighter plug that has a usb input for charging the device. You would need to use your own usb cable to plug in the phone- I'd just get an extra to keep in the car. Check it out: http://www.griffintechnology.com/products/itrip-auto-universal-plus
I had this for a while, but was very unimpressed by the FM transmission power. Amazon.com: Scosche Digital FM Transmitter and USB Charger: Electronics You'd just plug your USB->Micro USB cable into the USB port on the adapter.
Amazon.com: Blackberry Storm FM Transmitter / Internet Music Vehicle Adapter with integrated Car Charger - Gomadic Brand w/ TipExchange Technology: Electronics This is what I was thinking, anyone seen one cheaper for the droid?