I got me the 'AIO PREMIUM CAR HOLDER & CHARGER FOR NOKIA N900 UK' and its pretty good. Ordered it from ebay and arrived in a week in Spain. The quality is good and the phone fits well. Maybe the charger cable is a bit short but that was ok for me. The headphone socket is free as you can see in the pics.

I forgot to make a pic for portrait mode but that works great too. Let me know if more pics are needed.