By "aux out," are you looking specifically for line out? Because if you're not, you can just plug the cable into your headphone port and then just get any old iPhone car charger (I got Griffin's current model; works great).
For these cigarette chargers that have an aux out, how does calling work?
Right now when I use the line-out jack on the phone with my car, I hear the other person through the car's speakers and still speak to the phone's mic.
I remember reading that with the belkin one, it doesn't send your conversations through the dock's line-out and requires you to pick up your handset to answer calls.
I've just ordered the Bluetooth version of the LiquidAUX. It specifically is set up for calling in the car that I'm after:
http://www.amazon.com/Kensington-Li...sr_1?ie=UTF8&s=wireless&qid=1218896396&sr=8-1
The only thing I'm a bit concerned about is how well the other party will be able to hear me. The microphone on the kensington is on the cigarette adaptor...kind of far from my mouth in the car. However, I read a review that praised the call quality (both ways) saying all the other parties could hear them very clearly.