I got into the conclusion that the best method to use my UMPC in my car is going to be using parts made by RAM-Mount. This system is very flexible and will allow me to switch from a PDA cradle to a UMPC cradle very easily. I started by getting some of the pieces needed in my project and the first one to arrive was the Universal PDA Holder.
I'm very impressed by the quality of this holder and I hope that the quality of the holder for UMPC that they are about to release is the same. You can check more pictures here.