I couldn't find any posts here recommending a car mount for the N80, so I went ahead and bought this:
http://www.expansys.com/product.asp?code=136061
It works well. The holder will hold the phone well with the slide open or closed. It has a ratchet and spring arrangement with foamy grips, so you just pop the phone in and squeeze until it grips as tightly as you want. The grips don't interfere with the camera button. It's also adjustable at the bottom, meaning you can plug in a car charger while the phone is in the holder.
The kit I linked to comes with a car charger, and various ways of attaching the holder to your car. The holder itself will mount into your car's air vent. It does this in such a way as to leave a gap so it does not impede the airflow. But I found that the angle of the phone was not ideal.
There is also a self-adhesive attachment with ball and socket joint. It needs a flat surface to attach to, and my car's dashboard is all curves, so I didn't try this.
There is a sucker that sticks onto the windscreen with a bendy rod. This works much better than I expected. Once attached it will not come off (unless you want it to by releasing the lever). With this I was able to get the phone in just the right position, to the extent that I can reach out with a finger and press buttons with my hand on the wheel. The only downside is that it does wobble and vibrate with bumps in the road, but not so much that it is unusable. I found I could still see the map on my navigation software just fine.
Finally, this kit includes a car charger with an N80 style plug. Not bad for �17. You can also buy the parts separately.
Hope that helps someone.