This plugin gives a quick example of a generic parent child relationship custom post type.
Generic parent child custom post type is simply a plugin that gives an example of how to create two custom post types where one is the parent and you can assign any number child posts to it.
The two post types are called Parents and Children. As an example you could call them Car Dealership and Cars and assign a bunch of cars to a dealership.
There is an example template file included which displays the parent post as well as a list of all the children posts assigned to it.
It's really more of a starting point but it does work.
Requires WordPress 3.0 and PHP 5.